Emscripten support

[joemarshall]

Summary

I added a discussion for this ages back but there's been no input, so I've written it (because I was contracted to do the work anyway, so I might as well contribute it upstream). This PR adds support for running in emscripten / webassembly platforms, where all network connections go via the browser.

Currently in progress, but tests okay locally, so I've opened this to check the CI changes, I've got to update docs also.

Checklist

• [X ] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[tomchristie]

This is really interesting, thanks... ☺️

I've taken a bit of a look at the ecosystem here, tho am going to need a bit more orientation... Would it make sense to document an example of how to write an HTML page that includes a Python REPL with httpx imported and available?

[tomchristie]

Thanks again for your work here @joemarshall.
Here's where I think we're at on this...

• Lazy load certifi & httpcore
• Import ssl under typechecking branches.
• Consider introducing JSFetchTransport().

[joemarshall]

@tomchristie I put in the PR that makes import ssl optional now (#3385 )

I updated this PR so it follows on from that PR.

How this PR works now is it moves _transports.default into _transports.httpcore, which defines [Async]HTTPCoreTransport, adds an extra module _transports.jsfetch file which defines [Async]JavascriptFetchTransport. Then in _transports/__init__.py it adds an alias of HTTPTransport which goes to whichever HTTP backend is in use (i.e. httpcore by default, JS fetch on emscripten)

[joemarshall]

@tomchristie I updated this to follow the changes in master - I think #3385 is redundant now, as the ssl changes are minimal at this point.