remove Cython extension from future Python-bundle-PyPI easyconfigs

[smoors]

rationale:

• Cython should be a builddep not a dep in most cases
• some Python packages require a specific Cython version
• the installation Python-bundle-PyPI itself currently does not need Cython

it's probably too risky to do this for the existing Python-bundle-PyPI easyconfigs, but i want to make sure the next version does not have Cython in it (or only as a builddep if needed).

[WilleBell]

This would be useful for #20525 where Cython had to be included as a dependency after Python-bundle-PyPI because sci-kit-learn v1.4.2 requires a newer Cython version to build. Same workaround was used for Cartopy-0.22.0-foss-2023a.eb . They workaround works, but having 2 versions in the path even though only 1 is used doesn't seem ideal. Neither is having a dependency that is only needed as build dependency.