Improve compatibility of `scripts/check-docker.py`

[achimnol]

Currently install-dev.sh executes it using $bpython (aka "bootstrap Python"), which may be a system Python.So if the system Python is older than 3.7 or does not allow silent installation of requests and requests_unixsocket, the script will break.

Let's do:

• check-docker.py: Write a minimalistic replacement of requests and requests_unixsocket with the Python standard library only. Rewrite the script using those new wrappers.
• install-dev.sh: Use $(pyenv prefix $(pyenv latest 3.9))/bin/python to execute check-docker.py, as 3.9 is required as the pants runtime.

JIRA Issue: BA-218

[achimnol]

This replaces #612.