WIP: Add python+coverage example

[TLATER]

Currently WIP because:

• $(location) and $(rootpath) make vars aren't working correctly
  • This is what we tried to fix with the
    PYTHON_COVERAGE_TARGET variable, which was ultimately not accepted. See also next point.
• Does not use the new toolchain feature that supports adding coveragepy directly to the python toolchain
  • Added together with the new coveragepy support in rules/python: Add a coverage_tool attribute to py_runtime. bazelbuild/bazel#15590, and is probably better to use than the $(rootpath) and pip_install solution we're showing here
  • While I'd love to rewrite the blog post to show this instead, I'd need a bit more time for that (at least a few hours). I'll see what Richard says, but I'm unsure I can make time for this right now.
• Python imports are wonky
  • For some reason, the sibling module can only be imported under a parent python module. I confirmed this by looking through PYTHON_PATH
  • Seems to be a Bazel regression? Strangely nobody is complaining about this yet, we should probably raise an issue
• Not tested in remote execution
  • No access to an engflow setup to test this right this instant
  • Since we're not using a toolchain (and even if we were we're using pip_install), this requires a python on the host, so there's a decent chance it won't work
• Requires bumping Bazel to a version with rules/python: Add a coverage_tool attribute to py_runtime. bazelbuild/bazel#15590 merged, which currently is only available in prereleases