Add support for Python 3.10

[hugovk]

Python 3.10 is due out in a couple of weeks, on 2021-10-04.

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

• Add Trove classifier
• Test 3.10-dev on Ubuntu and macOS
  • Skip Windows until lxml has a binary wheel available
• Bump pinned dependencies where the old version doesn't support 3.10
• Remove redundant mock backport and some old 3.5 code

[hugovk]

Updated now Python 3.10 final is out.

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

[ulgens]

Any update on this?

[mvantellingen]

Merged, thanks :)