Export typing information

[JanLuca]

This commit adds the flag that the package contains typing
information. Therefore other packages can use the typing data to check their
code by mypy.

Ref.: https://mypy.readthedocs.io/en/stable/installed_packages.html#making-pep-561-compatible-packages

Signed-off-by: Jan Luca Naumann [email protected]

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[JanLuca]

@googlebot I signed it!

[alewis]

Looks good!

[mganahl]

Thanks for the PR! I'll fix the build and merge this.

[codecov-commenter]

Codecov Report

Merging #923 (357f430) into master (20dc783) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #923   +/-   ##
=======================================
  Coverage   98.04%   98.04%           
=======================================
  Files         129      129           
  Lines       22394    22394           
=======================================
  Hits        21957    21957           
  Misses        437      437           

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)

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