Add explicit testing of all supported Python versions

[PProfizi]

Fixes support for vtk>=9.1.0.
Makes Python 3.10 available.
There is still a problem with Python 3.10 on Ubuntu-18.04, see #416.

[codecov]

Codecov Report

Merging #403 (97c8f16) into master (b6d1186) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #403      +/-   ##
==========================================
+ Coverage   86.30%   86.57%   +0.27%     
==========================================
  Files          61       61              
  Lines        6701     6718      +17     
==========================================
+ Hits         5783     5816      +33     
+ Misses        918      902      -16     

[PProfizi]

R222 Python3.7 Ubuntu-18.04:
test_python_plugins.py tests fail due to hashlib import, apparently because of it not finding openssl.
A problem with the provided python environment for test_python_plugins?

@cbellot000 Strangely this is a problem only for Python 3.7 on Ubuntu-18.04.

[PProfizi]

R222 & R221 Python3.10 Windows & Ubuntu-18.04:
A vtk==9.0.3 requirement is in requirements_test.txt but not in the setup.py. Will need to remove this. This might trigger issues discussed here: ansys/pydpf-post#132.

[PProfizi]

Run_examples Python3.* Ubuntu-18.04:
The Install OpenGL step must be changed to the pyvista/setup-headless-display action to be compatible with Linux.

[cbellot000]

@PProfizi could you please either make a doc or setup a meeting (maybe with a slide on which pipelines are available) to decide when each pipeline is ran?