RELEASING: add pre-release suggestion to test downstream projects

[bluetech]

The recent 1.1.0 release broke two different users of pluggy (devpi and conda). In order to avoid such things in the future, we should improve our pre-release testing.

I went through pluggy's reverse dependencies and found a few large projects which use pluggy (at least these are the ones I recognized). Here is the list, with notes on how to test them (I haven't verified the steps yet):

• airflow
  • To test: TODO
• conda
  • To test: TODO
• datasette
  • To test:
    • pytest -m venv venv
    • venv/bin/pip install -e .[test] https://github.com/pytest-dev/pluggy/archive/refs/heads/main.zip
    • venv/bin/pytest
• devpi
  • To test: TODO
• hatch
  • To test:
    • pytest -m venv venv
    • venv/bin/pip install -e . -e ./backend https://github.com/pytest-dev/pluggy/archive/refs/heads/main.zip
    • venv/bin/hatch run full
• pytest:
  • To test:
    • tox -e py-pluggymain
  • pytest continuously tests pluggy main in its CI
• tox
  • To test: TODO

For now we can just suggest testing these manually in the RELEASING file, but in the future maybe we can automate it.