Update to pytest 8

https://docs.pytest.org/en/latest/changelog.html

Release notes for pytest 8 were written mostly for 8.0.0rc1 and 8.0.0rc2
instead of 8.0.0. 8.1.0 was yanked so 8.1.1 will be the minimum version.

pyproject.toml

@@ -50,7 +50,7 @@ tests = [
  "coverage[toml]>=7,<8",
  "freezegun>=1,<2",
  "httpx>=0.23,<1",
- "pytest>=7,<8",
+ "pytest>=8.1.1,<9",
  "pytest-mock>=3,<4",
 ]