Ease testing of local stubs by extending "MYPYPATH"

[Delgan]

Hi!

I would like to propose a fix for #17.

I'm using a stub file and I noticed that pytest-mypy-plugins did not work well in the following scenarios:

• Installing a local library with pip install -e .
• Running pytest without prior installation of the local library
• Updating the local stub file after installation with pip intall .

I think possible workarounds have been implemented (like importing mypy_path and MYPYPATH), but they don't seem to work out of the box. Any reason not to automatically add the project sources to paths searched by Mypy?

[Delgan]

@sobolevn I cloned the two repository locally, applied my patch manually, and ran the tests suite. Both succeeded. :+1:

[sobolevn]

Thanks for doing this!