Match dotfiles in sdist include/exclude glob patterns

[aopv]

Summary

Sdist include and exclude patterns were expanded using the stdlib glob module. glob's wildcards do not match files whose names start with a dot, so a recursive pattern such as:

[tool.flit.sdist]
exclude = ["**/*.swp"]

failed to exclude editor swap files like .foo.py.swp from the sdist, even though the docs advertise recursive ** support. As reported in #746, an explicit full path to a dotfile did exclude it, but a ** glob pattern did not, because glob skips dotfiles by default.

Fix

flit_core/flit_core/sdist.py now expands the patterns with pathlib.Path.glob instead of glob.glob(..., recursive=True). Path.glob matches dotfiles by default and preserves the same recursive ** semantics (including matching entries in the base directory). This deliberately avoids glob's include_hidden keyword, which is only available on Python 3.11+, so the fix works on flit_core's minimum supported Python (3.8).

Tests / Verification

• Extended the inclusion sample fixture with a hidden file doc/subdir/.hidden.swp and an exclude entry doc/**/*.swp, and added an assertion to test_include_exclude that the dotfile is excluded.
• Confirmed the new assertion fails on the unpatched code (the dotfile was still included) and passes after the fix.
• Ran the full flit_core test suite (flit_core/tests_core/) plus the top-level tests/test_sdist.py: all green (218 passed, 1 pre-existing skip).
• Added a doc/history.rst entry.

Disclosure: prepared with AI assistance; reviewed and verified locally.