env: Add surrogate for pytest.deprecated_call for ptyest<3.9

[EricCousineau-TRI]

Description

Resolves #2922

Suggested changelog entry:

Testing only; doesn't merit a change?

[EricCousineau-TRI]

@YannickJadoul or @rwgk Any chance y'all have time for a quick stamp?

[EricCousineau-TRI]

I'd also be fine with telling pybind11 to fail fast if an old version of pytest is found (e.g. require >=3.9).

Lemme know!

[rwgk]

I'd also be fine with telling pybind11 to fail fast if an old version of pytest is found (e.g. require >=3.9).

Lemme know!

I just looked, pytest 3.9 was released in October 2018, nearly 2.5 years ago.
My vote: require >=3.9
In the interest of simplicity/clarity/long-term maintenance.

[EricCousineau-TRI]

Er, only caveat is that Ubuntu 18.04 also provisions pytest==3.3.2 for CPython 3.6.9 :(

$ dpkg -S $(which python3.6)
python3.6-minimal: /usr/bin/python3.6
$ python3.6 -m pytest --version
This is pytest version 3.3.2, imported from /usr/lib/python3/dist-packages/pytest.py

In that case, it'd be requisite that testing be done inside a venv per the CONTRIBUTING instructions.

We OK making that constraint? (and do we do that in CI?)

[EricCousineau-TRI]

Reviewing CI, seems like yeah, venv is always used. Kewl, will now constrain.

EDIT: Er, easy to do in Python code, less trivial in tracing back through CMake logic. Will need to revisit tomorrow or next week :(

[henryiii]

I like this solution; unlike a normal Python package, we are not able to require Python things; so being able to use pytest in the default environment for reasonably recent OS's (Ubuntu 18.04) is nice.

[henryiii]

I think we used to use the default pytest, and this got changed in the CI to force a newer pytest. As long as it's clearly marked to be removed when we do bump the minimum pytest, I think it's fine. This is useful for deploying pybind11 in package managers, where they generally use the system packages for dependencies.

[rwgk]

In support of Henry's approval.

[EricCousineau-TRI]

Gotcha, thank ya for the rationale and and history - and happy to merge in with less work haha.