Skip checking GitHub in linkcheck

[hugovk]

linkcheck failed because GitHub rate limits us and then the workflow times out.

-rate limited-   https://github.com/python/cpython/blob/main/Doc/README.rst | sleeping...
-rate limited-   https://github.com/python/cpython/blob/main/Doc/Makefile | sleeping...
-rate limited-   https://github.com/python/cpython/blob/main/Grammar | sleeping...
-rate limited-   https://github.com/python/cpython/blob/main/Doc/tools/extensions/c_annotations.py | sleeping...
Error: The operation was canceled.

Shall we skip checking GitHub in linkcheck?

Originally posted by @hugovk in #1552 (comment)

[StanFromIreland]

This can be closed, #1556 was merged.

[hugovk]

Thanks for the reminder.

Tip: you can use "Fixes #xxx" in PRs to autoclose issues on merge.

[StanFromIreland]

Yes I forgot in that one.

Closes XXX works too.