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 #763 to remind us to revert this change when possible.

[codecov]

Codecov Report

Merging #761 (87c2105) into master (402b8e6) will decrease coverage by 6.94%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #761      +/-   ##
==========================================
- Coverage   82.45%   75.50%   -6.95%     
==========================================
  Files         107       72      -35     
  Lines       12658     8243    -4415     
==========================================
- Hits        10437     6224    -4213     
+ Misses       2221     2019     -202     

Impacted Files	Coverage Δ
...i/explainers/anchors/anchor_tabular_distributed.py	27.18% <ø> (ø)
alibi/utils/tests/test_distributed.py
alibi/explainers/tests/test_anchor_image.py
alibi/utils/tests/test_import_optional.py
alibi/tests/test_dep_management.py
...rs/tests/test_simiarlity/test_grad_methods_unit.py
alibi/explainers/tests/test_anchor_tabular.py
alibi/utils/tests/test_mapping.py
alibi/datasets/tests/test_datasets.py
alibi/explainers/tests/test_cem.py
... and 26 more