Replace custom mypy plugin for decorators with ParamSpec

[sobolevn]

Closes #823

[codecov]

Codecov Report

Merging #1196 (689b84d) into master (57be41c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1196   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           79        79           
  Lines         2414      2414           
  Branches       208       208           
=========================================
  Hits          2414      2414           

Impacted Files	Coverage Δ
returns/functions.py	100.00% <100.00%> (ø)
returns/future.py	100.00% <100.00%> (ø)
returns/io.py	100.00% <100.00%> (ø)
returns/maybe.py	100.00% <100.00%> (ø)
returns/result.py	100.00% <100.00%> (ø)

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