Temporary fix for mypy issue in Python 3.10 CI

[ascillitoe]

A mypy error occurs due to a numpy file with mypy 0.97x and the new Python 3.10.7 (see python/mypy#13627). This will hopefully be fixed by mypy 0.980 or mypy 0.990. Until then, this PR explicitly sets the CI to use Python 3.10.6.

I've opened issue #620 to remind us to revert this change when possible.

[codecov-commenter]

Codecov Report

Merging #619 (7677796) into master (0d46a04) will increase coverage by 3.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #619      +/-   ##
==========================================
+ Coverage   74.63%   77.95%   +3.31%     
==========================================
  Files         123      123              
  Lines        8749     8749              
==========================================
+ Hits         6530     6820     +290     
+ Misses       2219     1929     -290     

Flag	Coverage Δ
ubuntu-latest-3.10.6	77.34% <ø> (?)
ubuntu-latest-3.8	77.79% <ø> (?)
windows-latest-3.9	74.47% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
alibi_detect/utils/_types.py	93.10% <0.00%> (-3.45%)	⬇️
alibi_detect/saving/tensorflow/_saving.py	75.52% <0.00%> (+0.89%)	⬆️
alibi_detect/saving/tensorflow/_loading.py	83.89% <0.00%> (+1.12%)	⬆️
alibi_detect/cd/base.py	90.93% <0.00%> (+1.67%)	⬆️
alibi_detect/utils/frameworks.py	91.66% <0.00%> (+4.16%)	⬆️
alibi_detect/cd/learned_kernel.py	100.00% <0.00%> (+4.34%)	⬆️
alibi_detect/cd/mmd.py	97.72% <0.00%> (+6.81%)	⬆️
alibi_detect/cd/keops/learned_kernel.py	75.00% <0.00%> (+75.00%)	⬆️
alibi_detect/od/prophet.py	88.37% <0.00%> (+86.04%)	⬆️
alibi_detect/cd/keops/mmd.py	96.36% <0.00%> (+96.36%)	⬆️
... and 2 more