Update semver requirement in /requirements

Updates the requirements on [semver](https://github.com/python-semver/python-semver) to permit the latest version.
- [Release notes](https://github.com/python-semver/python-semver/releases)
- [Changelog](https://github.com/python-semver/python-semver/blob/master/CHANGELOG.rst)
- [Commits](python-semver/python-semver@2.13.0...3.0.2)

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

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

requirements/testing.txt

@@ -3,4 +3,4 @@
 
 pytest-cov>=3,<4.2
 pytest-qgis>=1.3,<2.1
-semver>=2.13,<2.14
+semver>=2.13,<3.1