DM-55493: Import pytest unconditionally in fastapi_safir_app conftest

[jonathansick]

Summary

In fastapi_safir_app/{{cookiecutter.name}}/tests/conftest.py the pytest
import sits inside a UWS-only Jinja conditional:

{% if cookiecutter.flavor == "UWS" -%}
import pytest
{% endif -%}
import pytest_asyncio

Since 9f67f2b, though, pytest is used in every flavor —
pytest_addoption(parser: pytest.Parser), @pytest.fixture def data(...),
and pytest.FixtureRequest. The committed non-UWS example shows the
result: example/tests/conftest.py calls pytest.Parser and
@pytest.fixture with no import pytest anywhere in the file.

A freshly generated non-UWS app fails at collection:

NameError: name 'pytest' is not defined

This moves the import out of the conditional.

Validation

• scons re-rendered the examples. The diff is exactly what it should be:
  example/tests/conftest.py gains import pytest, and
  example-uws/tests/conftest.py is unchanged — the UWS rendering
  stays byte-identical.
• templatekit check (with the two technote.toml ignores CI uses)
  passes: ✅ Passed!

References

Found while scanning template changes for the lsst-sqre maintenance
catalog. Ticket: DM-55493.

Companion PR: #353 fixes the sibling problem in square_pypi_package
(conftest imports safir, but the dev extra doesn't include it). The two
are independent.

[rra]

Just FYI, this is also fixed in #350, which is waiting on a code review. Apologies for missing that; I didn't generate and test a package after making the previous change. (We really should do that as part of the test suite. I keep thinking about it and then not finding the time to do so.)

[jonathansick]

My maintenance bot picked these up when I was doing upgrades in my apps, so no worries. I'll close this one and we can just go ahead and merge #350, which is where we want to go anyways.