test: migrate to pytest - #669

This is the only change I am unsure about:
slapdtest is packaged, e.g. in PyPI or Debian python3-ldap in the main distribution.
But adding a dependency on pytest in the core-library is not correct.
I added a extra-dependency for the project:

[project.optional-dependencies]
slapdtest = ["pytest"]

Debian could e.g. add some binary package python3-ldap+slapdtest (that's how they manager rust-extras - don't know if that's a valid Python-way).

Or they can split the binary package and make a python3-slapdtest - should be doable with less dh-python effort.
I don't know about other distributions.

So, I could also add a deferred import or try-except-ImportError construct.
Opinions?

Sure, for Fedora, I'd keep slapdtest in python3-ldap and add a python3-ldap+slapdtest extras metapackage to pull in pytest, following something like Hypothesis's packaging did.

I'd also move import pytest into the skip branch of skip_unless_ci(), so consumers using just SlapdObject don't need pytest. NumPy uses a similar deferred import in its test runner.

Would you be okay with renaming all files to test_-prefix?

Yeah, test_ makes sense and that's how pytest also discovers things. So I would remove the custom python_files = t_*.py setting and would update the explicit file paths in tox.ini along with the rename.
@mistotebe WDYT?

OK, I added an additional commit for the rename.

I think you might forgot to push it :)

Correct, I only pushed it into my local fork. Now it's there.