Make package PEP 561 compliant by including py.typed file

[jonded94]

Description

Using this package in a codebase with mypy unfortunately leads to errors like these:

error: Skipping analyzing "approvaltest.[...]": module is installed, but missing library stubs or py.typed marker  [import-untyped]

This is because this package is not properly PEP 561 compliant.

The solution

Adding a py.typed file since this package already large has type hints, it just wasn't marked as being typed.

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[jonded94]

@isidore any updates on this? This should be a very small, unproblematic change. Let me know what you think.