Update dmypy/client.py: Enable ANSI color codes for windows cmd

[wyattscarpenter]

I still use windows cmd, and the color codes emitted by dmypy do not work on there. instead printing a bunch of codes like ←[37m ←[39;49;00ms. However, for whatever reason you can fix this simply by calling os.system("") once. (The main mypy program works fine, presumably because it makes an os system call somewhere before it prints.)

I did not write a test of this, as that seems difficult and unnecessary. Instead, I manually tested it, and it worked great.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[hauntsaninja]

I think in mypy proper we do this with the initialize_win_colors thing in mypy util

[wyattscarpenter]

Thanks for the info! However, I've now tried importing and calling initialize_win_colors in various ways, from directly running the kernel32.SetConsoleMode function to instantiating the FancyFormatter class, but it actually seemed to have the opposite effect: it gets rid of the ansi codes, but just displays plain, black-and-white text. Regular mypy continues to work as normal.

[hauntsaninja]

Okay well, was worth a look, thank you! Merging this, but if someone knows what main mypy is doing, would be good to share code