Support python 3.10

[leszekhanusz]

• Add Python 3.10 tests on tox and on GitHub actions
• Update websockets to 10 for Python versions >= 3.6
• Update pytest related dependencies
• Update mypy dependency + add new required types dependencies
• Ignore new "There is no current event loop" warnings

[codecov-commenter]

Codecov Report

Merging #264 (3511c5b) into master (eb986df) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #264   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         1753      1763   +10     
=========================================
+ Hits          1753      1763   +10     

Impacted Files	Coverage Δ
gql/client.py	100.00% <100.00%> (ø)
gql/transport/websockets.py	100.00% <100.00%> (ø)

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