gh-36449: Autogenerate flint headers
We
- write down a script to auto-generate flint header files (currently at
https://github.com/videlec/flint-header-sage-autogen)
- replace most of the files in `src/sage/libs/flint` by auto-generated
files. Doing so, the custom sage code in `src/sage/libs/flint` is always
contained in files suffixed by `_sage.pxd`/`_sage.pyx`.
- deprecate the files in `src/sage/libs/arb/`
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
#35848
#### Upstream issues and patches
- https://github.com/flintlib/flint/pull/1523
- https://github.com/flintlib/flint/issues/1529
- https://github.com/flintlib/flint/pull/1532
- https://github.com/flintlib/flint/issues/1535
- https://github.com/flintlib/flint/pull/1536
- https://github.com/cython/cython/issues/5779
- https://github.com/flintlib/flint/issues/1653
- https://github.com/flintlib/flint/pull/1655
URL: https://github.com/sagemath/sage/pull/36449
Reported by: Vincent Delecroix
Reviewer(s): Marc Mezzarobba, Matthias Köppe, Vincent Delecroix
