Change `-m pip-run` in README to `-m pip_run`

[jwodder]

Recent versions of Python (not sure when it started; 3.9.0? 3.9.1?) no longer support using the -m option with an invalid module name. This PR thus updates the documentation to invoke pip-run with -m in a way that is guaranteed to work with all Python versions.

[jaraco]

Oh, this makes me sad. I really despise typing underscores as they're a chorded keystroke on my keyboard (and most users for that matter). I observe python -m pip-run still works on Python 3.9.0, so it's probably 3.9.1.

Hmm. It also works fine on Python 3.9.1:

$ docker run -it jaraco/multipy-tox
root@67e28fed58bd:/# python -V
Python 3.9.1
root@67e28fed58bd:/# python -m pip-run -- -c 'print("worked")'
worked

[jaraco]

By my reading, the only two changes to Python's runpy module in the past year are python/cpython#18699 and python/cpython#19239.

[jaraco]

Neither of those seem to be relevant. Can you provide a repro describing the behavior you encountered? Can you confirm that in your environment the pip-run.py script is installed as intended?

[jwodder]

• I am using Python 3.9.1 on macOS 11.2.1, installed with Homebrew.
• pip-run is version 8.4.1.
• pip is version 21.0.1.
• Running python3 -m pip-run --help fails with "/usr/local/opt/python@3.9/bin/python3.9: No module named pip-run", while spelling it with an underscore succeeds.
• The output from python3 -m pip show -f pip-run includes pip-run.py, but pip-run.py does not exist in /usr/local/lib/python3.9/site-packages/ (though pip_run does exist there).
• If I uninstall & reinstall pip-run, the pip-run.py file is now present.
• The wheel file for pip-run 8.4.1 includes a pip-run.py file.
  • (Side note: I see that the examples/ directory is included in the wheel. Did you mean to do that?)
• Everything works fine if I try installing pip-run in a python:3.9 Docker container with pip 21.0.1.

So I guess just close this as "can't reproduce"?

[jwodder]

Interestingly, I just used Homebrew to upgrade Python to 3.9.2, and the pip-run.py file disappeared! I think I see what's causing it, and I've filed this as a bug with Homebrew.