GitHub Actions: Add Python 3.10 to the testing

[cclauss]

https://pythoninsider.blogspot.com/2021/10/python-3100-is-available.html

"3.10" must be quoted in YAML. https://dev.to/hugovk/the-python-3-1-problem-85g

[codecov]

Codecov Report

Merging #605 (8655dc8) into master (e9b6edf) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##            master      #605     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        21     +10     
  Lines         1020      2412   +1392     
  Branches       201       533    +332     
===========================================
+ Hits          1020      2412   +1392     

Flag	Coverage Δ
unittests	99.91% <99.89%> (?)

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

Impacted Files	Coverage Δ
datamodel_code_generator/model/__init__.py	100.00% <ø> (ø)
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
...atamodel_code_generator/model/pydantic/__init__.py	100.00% <100.00%> (ø)
...amodel_code_generator/model/pydantic/base_model.py	100.00% <100.00%> (ø)
... and 18 more

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[koxudaxi]

@cclauss
Thank you for creating this issue.
pydantic hasn't supported Python3.10 yet.
We must wait for the next release.
pydantic/pydantic#2885