Test Python 3.10 release candidate

[hugovk]

Python 3.10.0 final is due for release in October:

• https://www.python.org/dev/peps/pep-0619/

The first release candidate is now out and the Python release team has issued a call to action for community members:

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.10 compatibilities during this phase. As always, report any issues to the Python bug tracker.

https://discuss.python.org/t/python-3-10-0rc1-is-now-available/9982?u=hugovk

So let's also test 3.10 on the CI. The good news is everything passes.

---

I didn't yet add 3.10 to python_requires, classifiers or README.rst, that can wait until 3.10 is released and officially supported, but could be updated now if you wish.

[codecov]

Codecov Report

Merging #166 (4497019) into master (0041e1d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #166   +/-   ##
=======================================
  Coverage   94.85%   94.85%           
=======================================
  Files          10       10           
  Lines         350      350           
  Branches       71       71           
=======================================
  Hits          332      332           
  Misses         11       11           
  Partials        7        7           

---

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 0041e1d...4497019. Read the comment docs.

[rayrrr]

Hi @hugovk thank you for this. My recommendation would be for you to finish adding 3.10 to python_requires, classifiers and README.rst. I've converted this PR to a Draft; we can convert it back to a regular PR and merge it when 3.10 is released. Sound good?

[hugovk]

Sounds good, and done!

[rayrrr]

Converting this back to draft for now. According to https://www.python.org/dev/peps/pep-0619/#release-schedule Python 3.10 is scheduled for release in October, 2021-10-04 to be exact, so we can merge it at that point.

[hugovk]

Exactly, the idea of this PR was to test against the release candidate right now.

That way if there's any problems found, either in this library, or in CPython itself, they can be fixed before 3.10.0 final is out.

The Python release team has even asked to test now:

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.10 compatibilities during this phase. As always, report any issues to the Python bug tracker.

Then when 3.10.0 final is out, the library can test against it and declare full support (via classifiers etc.).

---

(As an aside, a number of libraries are releasing for 3.10 already, because the RC should be stable, and it helps others test their libraries e.g. numpy/numpy#19630, but it's less important for pure Python libraries such as this.)

[hugovk]

3.10 is now out! Ready for review!

https://discuss.python.org/t/python-3-10-0-is-now-available/10955

[swainn]

When is the next planned release to PyPI? It would be great to have a version that supports Python 3.10 out.