Address issues in testing framework

[azogue]

closes #10 #11

Changes

• Change to poetry_core
• Unpin versions for flake8 plugins, dev-dependencies, and python version
• Add mypy to pre-commit, and address typing issues
• Move to pytest >= v6, and change pytest-aiohttp for pytest-asyncio to run async tests.

[codecov-commenter]

Codecov Report

Merging #13 (16b102d) into master (56fc6da) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #13   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          168       170    +2     
  Branches        31        31           
=========================================
+ Hits           168       170    +2     

Impacted Files	Coverage Δ
aiopvpc/pvpc_data.py	100.00% <100.00%> (ø)

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