Add Python 3.15 urllib.parse updates

[JelleZijlstra]

Summary

• Add Python 3.15 urllib.parse result geturl methods.
• Model missing_as_none result objects with optional missing components.
• Add keep_empty keyword support for urlunparse and urlunsplit.
• Remove corresponding 3.15 stubtest allowlist entries.

Relevant report entries

• Added urllib.parse missing_as_none and keep_empty options (CPython PR: urllib.parse wrongly strips empty #fragment, ?query, //netloc cpython#67041; source: https://github.com/python/cpython/blob/main/Lib/urllib/parse.py)

[JelleZijlstra]

This feels messy but I haven't looked too closely. Maybe we need generics? I'm going to focus on the rest of the stdlib first, but if anyone has strong opinions feel free to take a look.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉