Add typed marker (PEP561)

[eggplants]

https://www.python.org/dev/peps/pep-0561/#id13

In order to make packaging and distributing type information as simple and easy as possible, packaging and distribution is done through existing frameworks.

Package maintainers who wish to support type checking of their code MUST add a marker file named py.typed to their package supporting typing. This marker applies recursively: if a top-level package includes it, all its sub-packages MUST support type checking as well. To have this file installed with the package, maintainers can use existing packaging options such as package_data in distutils, shown below.

[codecov]

Codecov Report

Merging #167 (3b31f93) into master (d6ef550) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #167   +/-   ##
=======================================
  Coverage   95.46%   95.46%           
=======================================
  Files          10       10           
  Lines         661      661           
=======================================
  Hits          631      631           
  Misses         30       30           

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