fixed StreamT annotations, TPorTopicSet can also be optional

[tworedz]

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Description

• Fixed / added type annotations mostly for StreamT class.
• Fixed generic argument for EventT class.
• Make sure that App.commit accepts None as a documentation saying (https://faust-streaming.github.io/faust/reference/faust.app.html#faust.app.App.commit)
• Removed __iter__/__aiter__ methods on StreamT. And it makes sense, because it is extending AsyncIterable, which already implements __aiter__. __iter__ was just redundant here and broke typing.

[tworedz]

please review @wbarnha

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.09%. Comparing base (ff75c0b) to head (a44deff).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #660   +/-   ##
=======================================
  Coverage   94.09%   94.09%           
=======================================
  Files         102      102           
  Lines       11100    11100           
  Branches     1196     1196           
=======================================
  Hits        10444    10444           
  Misses        557      557           
  Partials       99       99           

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[tworedz]

Thanks! Seems that pypy doesn't like my changes, btw. Is it because of me or they were failing before?