Switch to Simple API

[ryanking13]

This PR works, but it is for now blocked by: #62 (comment)

• Change PyPI endpoint from JSON API (https://pypi.org/pypi/{pkgname}/json) to Simple API (https://pypi.org/simple)
• Vendored part of https://github.com/brettcannon/mousebender, which handles Simple API response.

[rth]

So what do you think we should about this @ryanking13 ? Do you think we could maybe add Sample API support while keeping current behavior for PyPI for now, or that would be too complex? I mean it does sound like there could be some fastly cache related improvements pypi/warehouse#12214 (comment) but I don't really understand how blocking this situation currently is.

[ryanking13]

@rth Nice timing. I was about to start working on this PR again.

Do you think we could maybe add Sample API support while keeping current behavior for PyPI for now, or that would be too complex?

I think we can go with supporting both Simple and legacy JSON API, (hopefully) it won't be very complex.

I mean it does sound like there could be some fastly cache related improvements pypi/warehouse#12214 (comment) but I don't really understand how blocking this situation currently is.

I think now the cache issue is okay. I mean fastly issue is still not resolved, but I believe now all Simple JSON responses in PyPI contains the CORS header correctly, so we can use it in micropip.

[ryanking13]

Continued on #75.