Complete testing and documentation of Python 3.9 support

[jdufresne]

Document and add support for Python 3.9.

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

---

Testing was added in 7503ad7 but is now
included in all CI matrices and is documented as trove classifier.

Python 3.9 was released November 15, 2020.

[codecov]

Codecov Report

Merging #1222 (b2195c2) into master (7521ec8) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1222      +/-   ##
==========================================
+ Coverage   99.44%   99.50%   +0.06%     
==========================================
  Files          36       36              
  Lines        2863     2855       -8     
  Branches      336      332       -4     
==========================================
- Hits         2847     2841       -6     
+ Misses          9        8       -1     
+ Partials        7        6       -1     

Impacted Files	Coverage Δ
piptools/repositories/pypi.py	95.51% <0.00%> (+0.64%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1d09299...1c4cc11. Read the comment docs.

[atugushev]

Thanks 👍 I think it would be good to remove py35 (due to EOL) in a following up PR.

[jdufresne]

I think it would be good to remove py35 (due to EOL) in a following up PR.

Would be happy to.

What is the pip-tools policy on supported Pythons?

If we're using EOL as a guideline, then we should begin work towards dropping Python 2. Something I'm also happy to contribute to. See https://devguide.python.org/#status-of-python-branches

Or, on the other hand, are we trying to match the pip support? If so, they still support Python 3.5.

Or, is the policy drop EOL Pythons except 2.7?

I looked through the docs and wasn't able to find anything explicit. I could add something to https://github.com/jazzband/pip-tools#versions-and-compatibility once I understand the policy.

My preference is to follow Python EOL schedule, but I recognize that may be too fast for some projects that use pip-tools.