fix checking for pytest-xprocess

[mgorny]

Fix conftest not to require pytest_xprocess module. This module has been removed back in 2021, and it is missing since pytest-xprocess 0.19.0. Checking for just xprocess is fine, and makes all tests run again.

• fixes pytest-xprocess is not detected correctly since 0.19.0 #592

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[mgorny]

Ok, to get things in order: none of the CI failures are technically introduced by this PR.

pre-commit finds style issues in code that wasn't touched here. CI fails because of some existing redis-related failures; I suspect either these tests didn't run before, or something changed in the meantime.