Dependency on `asynccontextgenerator`

[thearchitector]

@asynccontextgenerator is available in the standard contextlib module from Python 3.7+, and available via package for 3.5 and 3.6.

This uses the package, but its not listed as a dependency. It would nice for it to be listed as one for specifically 3.5/6, via the python_version meta, with a corresponding check in the actual code to just use the std lib one if its available instead.

[Zac-HD]

https://github.com/python-trio/triopg/blob/ccf44f9b40f0dc1945b8dcd152deaba11f195b28/triopg/_triopg.py#L6

I'd prefer to replace this with the contextlib version, and explicitly drop support for end-of-life versions of Python via python_requires=">=3.8". @jakkdl?

[jakkdl]

on it!

[jakkdl]

oh boi, the test infra is horribly outdated.

[jakkdl]

on it!

dropping this. Writing the PR is fairly trivial, so if anybody does use triopg and want to update it and/or do other maintaining I'd recommend they do it. I'm instead creating #21