Add scripts to verify compatibility with multiple Python versions

[c0llab0rat0r]

Carved out of #257

Adds scripts that use Docker to verify the library's compatibility with the versions of Python it supports.

With this change, the travis build will still fail with style linting and type linting errors. Change #257 will correct the style linting errors, and this change will be rebased once that one is merged.

Once #257 and this change are both merged, the scripts in this change will fail on the first Python version, 3.5.4, because the current code base has syntax at

py-ipfs-http-client/ipfshttpclient/http_httpx.py

Line 48 in 52c1d2a

) -> RequestArgs:

that is not supported by 3.5.4 - this problem will be corrected by #258.

[c0llab0rat0r]

Build error for Windows appears transient:

# Daemon should not be running anymore
>   	assert not daemon_is_running()
E    assert not True
E     +  where True = <function test_daemon_stop.<locals>.daemon_is_running at 0x000001BD05C99820>()

[c0llab0rat0r]

./verify.sh 3.8.0 will pass.

./verify.sh 3.7.2 and 3.6.2 will fail, with problems related to flake8 and typing.Final not being present in versions of Python prior to 3.8.

[c0llab0rat0r]

Unrelated transient failure on existing build process on Windows; does not exercise new scripts which are only run manually and locally.

[c0llab0rat0r]

Added a commit to add support for Python 3.9 since that was added to the library dependencies and build recently.

Using these new scripts:

• Python 3.6.2 will fail - flake fails to load a flake plugin that uses typing.Final, which is not supported on Python 3.6
• Python 3.7.2 will fail - this library uses Literal which is not supported on Python 3.7
• Python 3.8.0 will pass
• Python 3.9.0 will pass

We might be using a version of flake that is too new to use with Python 3.6.

[c0llab0rat0r]

@ntninja

Transient windows failure

https://travis-ci.com/github/ipfs-shipyard/py-ipfs-http-client/jobs/498603490

[c0llab0rat0r]

Rebased from master; ready to go.

[ntninja]

Thanks! 🙂