Add support for Python 3.10

[hugovk]

Python 3.10 was released on 2021-10-04:

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

[codecov-commenter]

Codecov Report

Merging #58 (e1baf63) into master (6758ac6) will decrease coverage by 0.40%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
- Coverage   88.92%   88.52%   -0.41%     
==========================================
  Files          15       15              
  Lines        1626     1595      -31     
  Branches      309      309              
==========================================
- Hits         1446     1412      -34     
  Misses        124      124              
- Partials       56       59       +3     

Impacted Files	Coverage Δ
humanfriendly/terminal/__init__.py	72.54% <0.00%> (-1.89%)	:arrow_down:
humanfriendly/terminal/html.py	80.98% <0.00%> (-1.66%)	:arrow_down:
humanfriendly/terminal/spinners.py	85.71% <0.00%> (-0.59%)	:arrow_down:
humanfriendly/testing.py	92.30% <0.00%> (-0.52%)	:arrow_down:
humanfriendly/deprecation.py	95.52% <0.00%> (-0.13%)	:arrow_down:
humanfriendly/usage.py	93.87% <0.00%> (-0.13%)	:arrow_down:
humanfriendly/__init__.py	92.75% <0.00%> (-0.07%)	:arrow_down:
humanfriendly/prompts.py	96.26% <0.00%> (-0.04%)	:arrow_down:
humanfriendly/decorators.py	100.00% <0.00%> (ø)

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[hugovk]

Closing year-old PR, can re-open if interested and also add Python 3.11.