Add support for Heroku-20 (#968)

This adds support for the upcoming Heroku-20 stack.

The Heroku-20 Dockerfile is identical to that for Heroku-18, other
than the base image, and stack-related env var changes.

The initial Python versions made available will be those in:
https://devcenter.heroku.com/articles/python-support#supported-runtimes
https://devcenter.heroku.com/articles/python-support#supported-pypy-runtimes

...minus CPython 2.7, since it's EOL.

Which are:
* `python-3.6.12`
* `python-3.7.9`
* `python-3.8.6`
* `python-3.9.0`
* `pypy2.7-7.3.2`
* `pypy3.6-7.3.2`

Note: Unlike CPython 2.7, the PyPy 2.7 branch is still supported:
https://doc.pypy.org/en/latest/faq.html#how-long-will-pypy-support-python2

In addition, I've generated binaries for each patch release immediately
prior to the latest versions (with the exception of 3.9.0, since there
isn't one), otherwise it's not possible to run the "out of date Python"
warning tests.

The binaries were generated using the process here:
https://github.com/heroku/heroku-buildpack-python/blob/main/builds/README.md

Specifically:

```
make deploy-runtimes STACKS='heroku-20' \
  RUNTIMES='python-3.6.11 python-3.6.12 python-3.7.8 python-3.7.9 python-3.8.5 python-3.8.6 python-3.9.0 pypy2.7-7.3.1 pypy2.7-7.3.2 pypy3.6-7.3.1 pypy3.6-7.3.2' \
  ENV_FILE=...
```

Binaries for the GDAL/GEOS/PROJ feature have not been generated, since
it's deprecated and due for removal shortly:
https://help.heroku.com/D5INLB1A/python-s-build_with_geo_libraries-legacy-feature-is-now-deprecated

Note: Like the Python 3.9.0 release, this uses the new S3 bucket, so
apps will need to be using a recent version of the buildpack in order
to build on Heroku-20:
https://devcenter.heroku.com/articles/python-support#checking-the-python-buildpack-version

Closes @W-7485877@.

test/run-versions

@@ -14,6 +14,10 @@ testPythonVersionUnspecified() {
 }
 
 testPython2_7() {
+  # Python 2.7 is EOL, so it has not been built for Heroku-20.
+  if [[ $STACK == "heroku-20" ]]; then
+    return
+  fi
   compile "python2"
   assertCaptured "Installing ${LATEST_27}"
   assertCaptured "python-2-7-eol-faq";
@@ -24,6 +28,10 @@ testPython2_7() {
 }
 
 testPython2_7_warn() {
+  # Python 2.7 is EOL, so it has not been built for Heroku-20.
+  if [[ $STACK == "heroku-20" ]]; then
+    return
+  fi
   compile "python2_warn"
   assertCaptured "Installing python-2.7.15"
   assertCaptured "python-2-7-eol-faq";
@@ -33,6 +41,10 @@ testPython2_7_warn() {
 }
 
 testPython3_4() {
+  # Python 3.4 is EOL, so it has not been built for Heroku-20.
+  if [[ $STACK == "heroku-20" ]]; then
+    return
+  fi
   compile "python3_4"
   assertCaptured "Installing ${LATEST_34}"
   assertNotCaptured "security update"
@@ -44,6 +56,10 @@ testPython3_4() {
 }
 
 testPython3_4_warn() {
+  # Python 3.4 is EOL, so it has not been built for Heroku-20.
+  if [[ $STACK == "heroku-20" ]]; then
+    return
+  fi
   compile "python3_4_warn"
   assertCaptured "Installing python-3.4.9"
   assertCaptured "security update!"
@@ -54,6 +70,10 @@ testPython3_4_warn() {
 }
 
 testPython3_5() {
+  # Python 3.5 is EOL, so it has not been built for Heroku-20.
+  if [[ $STACK == "heroku-20" ]]; then
+    return
+  fi
   compile "python3_5"
   assertCaptured "Installing ${LATEST_35}"
   assertNotCaptured "security update"
@@ -63,6 +83,10 @@ testPython3_5() {
 }
 
 testPython3_5_warn() {
+  # Python 3.5 is EOL, so it has not been built for Heroku-20.
+  if [[ $STACK == "heroku-20" ]]; then
+    return
+  fi
   compile "python3_5_warn"
   assertCaptured "Installing python-3.5.6"
   assertCaptured "security update!"
@@ -81,7 +105,7 @@ testPython3_6() {
 
 testPython3_6_warn() {
   compile "python3_6_warn"
-  assertCaptured "Installing python-3.6.7"
+  assertCaptured "Installing python-3.6.11"
   assertCaptured "security update!"
   assertCaptured "${LATEST_36}"
   assertCapturedSuccess
@@ -106,7 +130,7 @@ testPython3_7_warn() {
     return
   fi
   compile "python3_7_warn"
-  assertCaptured "Installing python-3.7.1"
+  assertCaptured "Installing python-3.7.8"
   assertCaptured "security update!"
   assertCaptured "${LATEST_37}"
   assertCapturedSuccess
@@ -131,7 +155,7 @@ testPython3_8_warn() {
     return
   fi
   compile "python3_8_warn"
-  assertCaptured "Installing python-3.8.0"
+  assertCaptured "Installing python-3.8.5"
   assertCaptured "security update!"
   assertCaptured "${LATEST_38}"
   assertCapturedSuccess
@@ -208,7 +232,7 @@ testPypy2_7_warn() {
 testStickyPythonVersion() {
   local cache_dir="$(mktmpdir)"
   compile "python3_6_warn" "$cache_dir"
-  assertCaptured "Installing python-3.6.7"
+  assertCaptured "Installing python-3.6.11"
   assertCapturedSuccess
   compile "python_version_unspecified" "$cache_dir"
   assertNotCaptured "Installing python"
@@ -217,16 +241,16 @@ testStickyPythonVersion() {
   # Whilst this file seems like an implementation detail (so something that should
   # not be tested), we must guarantee the filename remains consistent for backwards
   # compatibility across buildpack versions for already-built apps.
-  assertFile "python-3.6.7" ".heroku/python-version"
+  assertFile "python-3.6.11" ".heroku/python-version"
 }
 
 testPythonVersionChange() {
   local cache_dir="$(mktmpdir)"
   compile "python3_6_warn" "$cache_dir"
-  assertCaptured "Installing python-3.6.7"
+  assertCaptured "Installing python-3.6.11"
   assertCapturedSuccess
   compile "python3_6" "$cache_dir"
-  assertCaptured "Found python-3.6.7, removing"
+  assertCaptured "Found python-3.6.11, removing"
   assertCapturedSuccess
 }