Add support for Python 3.11, drop EOL 3.6

[hugovk]

Follow on from #42, 2022 edition.

Python 3.11 was released on 2022-10-24 🚀

Python 3.6 is EOL since last year, and little used.

Here's the pip installs for python-multipart from PyPI for October 2022:

category	percent	downloads
3.7	31.66%	1,087,231
3.8	25.65%	880,742
3.10	14.68%	503,995
3.9	13.69%	470,078
null	13.22%	453,880
3.6	0.71%	24,449
3.11	0.40%	13,579
2.7	0.01%	336
3.5	0.00%	37
3.12	0.00%	11
3.4	0.00%	10
3.3	0.00%	3
Total		3,434,351

Source: pip install -U pypistats && pypistats python_minor python-multipart --last-month

Also:

• Add python_requires to help pip
• Replace deprecated unittest.makeSuite with unittest.defaultTestLoader.loadTestsFromTestCase (using pyupgrade)