Update pytest-xdist requirement from <3.4.0 to <3.7.0

Updates the requirements on [pytest-xdist](https://github.com/pytest-dev/pytest-xdist) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest-xdist/releases)
- [Changelog](https://github.com/pytest-dev/pytest-xdist/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-xdist@1.0...v3.6.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

requirements-dev.txt

@@ -22,7 +22,7 @@ pytest-benchmark
 pytest-env
 pytest-flakefinder
 pytest-timeout
-pytest-xdist < 3.4.0 # 3.5.0 introduces some flakiness. Need to investigate and resolve.
+pytest-xdist < 3.7.0 # 3.5.0 introduces some flakiness. Need to investigate and resolve.
 pytkdocs >= 0.14.2
 pyyaml
 redis>=5.0.1