bring back Python 3.7 support.

[T-256]

httpx still didn't drop it.
In Python 3.7 I can receive latest httpcore 0.17.3 but the latest httpx-ws 0.3.1 has problem with it (#35).
I need httpcore latest bugfixes so I can't pin it to compatible version for httpx-ws.
Currently I most use my fork, But if you want, we can push it to py37 branch? at least until httpx drop it.

[frankie567]

To be quite frank, no. Python 3.7 is now unsupported and put applications at risk. Besides, I can't afford personally to maintain a dedicated branch for compatibility.

I don't know what your requirements/constraints are, but I would say that you should try to upgrade to Python 3.8. Most likely, it won't break anything in your code.

[T-256]

My project based on Python 3.7 AST instructions (compiled codes e.g. .pyc files), I cannot parse Python 3.7 AST in Python 3.8.