Update cython requirement from <0.30,>=0.29.21 to >=0.29.21,<3.1 (#635)

Updates the requirements on [cython](https://github.com/cython/cython) to permit the latest version.
- [Release notes](https://github.com/cython/cython/releases)
- [Changelog](https://github.com/cython/cython/blob/master/CHANGES.rst)
- [Commits](cython/cython@0.29.21...3.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -1,5 +1,5 @@
 blessings>=1.7,<2
-Cython>=0.29.21,<0.30
+Cython>=0.29.21,<3.1
 defusedxml>=0.7.1,<1
 graphviz>=0.16,<0.21
 numpy>=1.19.5,<2