made dependencies on certifi and httpcore only load when required

[joemarshall]

As per #3330 this makes certifi and httpcore only load when required.

For now I haven't done the same with ssl, because SSLContext derives from ssl, which makes it a pain to load on demand. I can make that behind an ImportError guard, with a dummy SSLContext class, but it is a bit hacky. This is what asyncio does. The alterative is dynamically reloading the class and importing ssl on first instantiation (or overriding __new__ maybe), but again, it is a bit of a hack.

Personally I think there isn't a massive overhead to just making ssl a dependency even on platforms where it isn't being used, but I'm easy to go with the dummy class or something if you don't want that; I just worry that code may assume that the dummy SSLContext is a class derived from ssl.SSLContext and bad things will occur.

[joemarshall]

@tomchristie here's the change to lazy load httpcore and certifi

[tomchristie]

Nice work @joemarshall, thanks!

For the ssl case, this is probably okay for the implementation...

if ssl is None:
    class SSLContext:
        def __init__(self, ...):
            raise RuntimeError('...')
else:
    ...

With typechecking gates around the other cases.

[tomchristie]

😎