chore: nix shell: add python314

[fiforeach]

This PR adds Python 3.14 as an input to the Nix flake.

For instance, tests/fix_expected.py requires Python >= 3.14 (pathlib.Path.copy).

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 03a5db34c77db5266d21af56469ba43c9965d341 --onto cda84702e9b31165f1f83c657b532f36f34e0bd0. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-03 14:41:04)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 03a5db34c77db5266d21af56469ba43c9965d341 --onto 333ab1c6f0ce11f33551d6a736054cb72812cad9. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-06 10:03:39)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 03a5db34c77db5266d21af56469ba43c9965d341 --onto cda84702e9b31165f1f83c657b532f36f34e0bd0. You can force reference manual CI using the force-manual-ci label. (2026-03-03 14:41:06)

[fiforeach]

Ok, I removed Meld again since I think it is maybe not really a truly required dependency for development.