Adds support for:
* CPython 2.7.18, 3.5.9, 3.7.7 and 3.8.3
* PyPy 2.7 and 3.6, version 7.3.1
The binaries will need generating and uploading before CI will pass.
Note: Whilst the build script for CPython 3.8.3 did already exist in the
repository, it appears to have been accidentally created in #920, which
predated the existence of that version of Python - so the binaries do
not exist on S3.
The Heroku-18 Docker image tag has also been unpinned, since the new
libssl version is now available at runtime in all environments, so we
don't need to force building against the older version of the headers.
Fixes W-7582174.
Ed Morley