Emit metrics for how the Python version was chosen (#1069)

Currently an app's Python version can be set via a few different means:
- explicitly by the user (via `runtime.txt` or `Pipfile.lock`)
- implicitly via the sticky versions feature (for existing apps)
- implicitly via default version for new apps / those with empty cache

In order to determine the priority of features like automatic Python
patch version upgrades for sticky-versioned apps, it's useful to have
metrics for these.

There were previously no tests for either the sticky versions feature,
or changing the Python version by updating the `runtime.txt` file, so
I've added some now (given that I updated the conditional to add the
metrics, so useful to have coverage).

I've also removed the confusing overwrite of `DEFAULT_PYTHON_VERSION`
with the cached version, and kept them as two separate variables.

Closes @W-8099632@.
Closes @W-8099645@.

test/run-versions

@@ -214,6 +214,30 @@ testPypy2_7_warn() {
   fi
 }
 
+testStickyPythonVersion() {
+  local cache_dir="$(mktmpdir)"
+  compile "python3_6_warn" "$cache_dir"
+  assertCaptured "Installing python-3.6.7"
+  assertCapturedSuccess
+  compile "no-runtime-txt" "$cache_dir"
+  assertCaptured "Installing python-3.6.7"
+  assertCapturedSuccess
+  # Whilst this file seems like an implementation detail (so something that should
+  # not be tested), we must guarantee the filename remains consistent for backwards
+  # compatibility across buildpack versions for already-built apps.
+  assertFile "python-3.6.7" ".heroku/python-version"
+}
+
+testPythonVersionChange() {
+  local cache_dir="$(mktmpdir)"
+  compile "python3_6_warn" "$cache_dir"
+  assertCaptured "Installing python-3.6.7"
+  assertCapturedSuccess
+  compile "python3_6" "$cache_dir"
+  assertCaptured "Found python-3.6.7, removing"
+  assertCapturedSuccess
+}
+
 pushd $(dirname 0) >/dev/null
 popd >/dev/null