Update pytest-xdist requirement from ~=3.3 to ~=3.4 (#9231)

Updates the requirements on [pytest-xdist](https://github.com/pytest-dev/pytest-xdist) to permit the latest version.
- [Changelog](https://github.com/pytest-dev/pytest-xdist/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-xdist@v3.3.0...v3.4.0)

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

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requirements_test.txt

@@ -4,7 +4,7 @@ tbump~=6.11.0
 contributors-txt>=1.0.0
 pytest-cov~=4.1
 pytest-profiling~=1.7
-pytest-xdist~=3.3
+pytest-xdist~=3.4
 six
 # Type packages for mypy
 types-pkg_resources==0.1.3