build: Fix build reproducibility.

Without going the extra length of exporting `PYTHONHASHSEED=0` or
similar, the built sdist and wheel for a given version of the project
would have inconsistent random ordering of extras_require entries
between builds.

This was observed attempting to lock a VCS requirement on this project
here: pantsbuild/pants#21145
On the bright side, it led to Pex fixing its building and locking code
to be robust to this sort of (unintended) bad behavior:
pex-tool/pex#2554

setup.py

@@ -56,7 +56,7 @@ def load_requirements(file_list: Optional[Union[str, List[str]]] = None) -> List
 tsv_reqs = load_requirements("requirements/extra-csv.in")
 xlsx_reqs = load_requirements("requirements/extra-xlsx.in")
 
-all_doc_reqs = list(
+all_doc_reqs = sorted(
     set(
         csv_reqs
         + docx_reqs