Add use-dmypy option to MypyTool

[lukaspiatkowski]

Description

Mypy provides an API to use their daemon to speed up type checking. With this change, the use-dmypy configuration can be used to toggle usage of that API by prospector.

Motivation and Context

Mypy tends to be very slow on large repositories, using Mypy daemon makes it much faster.

How Has This Been Tested?

I've run prospector --with-tool mypy -t mypy <file> on a <file> in a large repository with and without use-dmypy option. The difference was 0.57s vs 17.44s in favor of using dmypy.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change requires a change to the dependencies
• I have updated the dependencies accordingly
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[carlio]

Also LGTM, so I shall merge this too.

Note that as a result of #612 the documentation is not building on readthedocs, I will have to look into that before making a release with these in.