Explicitly re-export names from __init__ module

[mthuurne]

By listing names intended for export in the __all__ module-level variable, static code checkers like mypy are able to verify that importing a name from a module that itself imported it is by design rather than by accident.

As flake8 understand __all__, this also removes the need for warning suppression comments.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.57%. Comparing base (fe1d0c6) to head (00e52e8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #591   +/-   ##
=======================================
  Coverage   98.57%   98.57%           
=======================================
  Files           6        6           
  Lines         772      773    +1     
=======================================
+ Hits          761      762    +1     
  Misses         11       11           

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[foarsitter]

I like the gradual improvements, thanks @mthuurne