Update pytest requirement from <8.0 to <9.0

Updates the requirements on [pytest](https://github.com/pytest-dev/pytest) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/main/CHANGELOG.rst)
- [Commits](pytest-dev/pytest@1.0.0b3...8.0.0)

---
updated-dependencies:
- dependency-name: pytest
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

pyproject.toml

@@ -76,7 +76,7 @@ dev = [
   "pre-commit >=2.9",
   "pybtex",
   "pylint",
-  "pytest <8.0", # Pinned due to breakage with xdoctest. See: https://github.com/Erotemic/xdoctest/issues/151
+  "pytest <9.0", # Pinned due to breakage with xdoctest. See: https://github.com/Erotemic/xdoctest/issues/151
   "pytest-cov",
   "pytest-socket",
   "pytest-xdist[psutil] >=3.2",