Add support for Windows and Cygwin

[kurtmckee]

This PR introduces the following changes:

• Add full support for Windows Powershell (CPython 3.8+) and Cygwin (CPython 2.7+, PyPy 2.7, and PyPy 3.9).
• Migrate all existing unit tests from bash to Python.
• Isolate all unit test filesystem interactions so they run entirely in a temporary directory. This includes verification that files are not being opened or created outside the temporary directory.
• Test dotbot in CI on Linux, MacOS, and Windows, across all supported CPython and PyPy versions.
• Add code coverage reports when running locally. This includes summary output at the CLI and generated HTML reports in htmlcov/.
• Document how to run the unit tests locally, including environment setup.

The changes have been tested on these platforms:

• Windows Powershell (locally, and in CI)
• Linux (locally, and in CI)
• Cygwin (locally)
• MacOS (in CI)

Closes #102 Closes #307 Closes #309

[eengstrom]

I am excited to see the new testing framework in action - it was always challenging to develop tests for new features. Thanks for the efforts @kurtmckee

[rgwood]

This looks like a huge improvement for dotbot on Windows, and I believe it will fix an issue I'm seeing that is closely related to #307. Thank you very much for all the work!

[anishathalye]

Thank you for this PR, looks awesome! I will take a closer look and give feedback / merge soon.

[kurtmckee]

Thanks all for the feedback!

@anishathalye Please let me know if you see anything that needs adjusting. I'm happy to make changes. Thanks for your work on dotbot, and for reviewing this PR!

[kurtmckee]

@anishathalye I've ported the recent repo changes over to keep this PR up-to-date. The latest commit resolves the associated merge conflicts.

[anishathalye]

This is beautiful, very thoroughly done. Thank you so much! And thank you for your patience, it took me forever to get around to looking at this.

[kurtmckee]

You're welcome, and thanks for all of your work on dotbot! 😀