Drop support for soon-EOL Python 3.6

[hugovk]

Changes proposed in this pull request:

• Drop support for Python 3.6, it's soon EOL
• Replace deprecated pypyX (which is also Python 3.6) with explicit pypy-X.Y
• Let's also make wheels for both PyPy3.7 and PyPy3.8
• Bump benchmarking from Ubuntu 18.04/Python 3.8 to Ubuntu 20.04/Python 3.10
• Cache pip using https://github.com/actions/setup-python instead of https://github.com/actions/cache
• Remove one instance of calling setup.py (calling it is deprecated) with https://github.com/pypa/build
• Delete unused scripts/build-manylinux.py

cycle	latest	release	eol
3.10	3.10.0	2021-10-04	2026-10-04
3.9	3.9.9	2020-10-05	2025-10-05
3.8	3.8.12	2019-10-14	2024-10-14
3.7	3.7.12	2018-06-27	2023-06-27
3.6	3.6.15	2016-12-23	2021-12-23

Here's the pip installs for ujson from PyPI for October 2021:

category	percent	downloads
3.7	78.51%	15,547,674
3.9	6.54%	1,294,795
3.8	6.12%	1,210,988
null	3.66%	724,945
3.6	3.66%	724,471
2.7	1.26%	248,535
3.5	0.16%	32,165
3.10	0.10%	19,036
3.4	0.00%	248
3.11	0.00%	26
3.3	0.00%	18
Total		19,802,901

Date range: 2021-10-01 - 2021-10-31

Source: pip install -U pypistats && pypistats python_minor ujson --last-month

TODO

• Merge Use cibuildwheel to build wheels. #491 first.
• When ready to merge, adjust the required CI status checks.

[codecov-commenter]

Codecov Report

Merging #490 (53a28fa) into main (6833495) will not change coverage.
The diff coverage is n/a.

❗ Current head 53a28fa differs from pull request most recent head bd66c08. Consider uploading reports for the commit bd66c08 to get more accurate results

@@           Coverage Diff           @@
##             main     #490   +/-   ##
=======================================
  Coverage   88.96%   88.96%           
=======================================
  Files           6        6           
  Lines        1685     1685           
=======================================
  Hits         1499     1499           
  Misses        186      186           

---

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 6833495...bd66c08. Read the comment docs.

[hugovk]

Will merge this and #491 in December, to coincide with 3.6 EOL month.

Will likely release before the actual EOL date, the previous 3.6-compatible versions will still be available on PyPI and python_requires will help out.