Support PEP517 in-tree build backend

[webknjaz]

I'm getting

ERROR: invocation failed (exit code 1), logfile: ~/src/github/ansible/pylibssh/.tox/.package/log/.package-2.log
=================================== log start ====================================
Traceback (most recent call last):
  File "~/.pyenv/versions/3.7.1/lib/python3.7/site-packages/tox/helper/build_requires.py", line 7, in <module>
    backend = __import__(backend_spec, fromlist=[None])
ModuleNotFoundError: No module named 'pep517_backend'

because of

$ cat pyproject.toml                            
[build-system]
...
backend-path = ["bin"]  # requires 'Pip>=20' or 'pep517>=0.6.0'
build-backend = "pep517_backend"

and

$ ls -l bin/pep517_backend.py      
-rw-r--r-- 1 me me 6.2K May  5 00:43 bin/pep517_backend.py

I think this should be considered a bug. Pip supports it starting v20 and pep517 supports it with v0.7 or so.

[gaborbernat]

I'll add it onto the tox 4 pile. Feel free to put in a PR if you want it for 3. I consider this more a feature request though as in-tree build backends were added later to PEP-517.

[webknjaz]

Yeah, I've done some research and it doesn't look super hard. Except that I cannot figure out how tox resets PYTHONPATH that I pass via env in calls to popen to "src".

[webknjaz]

@gaborbernat by the way, any reason to not rely on pep517 package from PyPI instead of having own implementation?

[gaborbernat]

@gaborbernat by the way, any reason to not rely on pep517 package from PyPI instead of having own implementation?

Speed, maintainability. pep517 is designed to run every time from scratch, and not to reuse the build environment. We have a good way to cache pyhton environments ourselves though.

[gaborbernat]

Released via https://pypi.org/project/tox/3.19.0/