Support Python 3.14t

[mdekstrand]

After basic Python 3.14 support (#880), we should also look towards free-threaded support.

• ENH: Backport free-threading support to 2.3 pandas-dev/pandas#61844
  • It would be appropriate to only fully support free-threading with Pandas 3.0 or later, once it is released
  • Making Pandas fully optional (Make Pandas optional #917) would also unblock this
• Add free-threading detection to lenskit doctor (lenskit doctor: detect free-threaded Python #996)
• Add 3.14t to test matrix
• Update Rust code to work with free-threading
• Enable PyPI package builds for free-threaded Python