gh-37520: Remove comment about python2 longs
<!-- v Describe your changes below in detail. -->
Python2 had long integer literals. For example `12L` was valid. In
python3 this is no longer valid.
This PR removes an outdated documentation that mentions this old
feature.
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/37520
Reported by: Eloi Torrents
Reviewer(s):
