sdist: match dotfiles in file pattern globbing

[DYNOSuprovo]

Fixes #746

Problem

When configuring sdist exclude patterns like exclude = ["**/*.swp"], standard glob.glob ignores hidden files (filenames beginning with ., such as .foo.swp created by editors like Vim) by default. As a result, those files remain included in sdist builds or cause untracked file errors during VCS checks.

Solution

• Switch FilePatterns in flit_core.sdist to expand patterns using pathlib.Path.glob, which matches hidden dotfiles across all supported Python versions.
• Normalize patterns for pathlib compatibility (e.g. converting **.ext to **/*.ext).
• Update default exclude in config.py from **.pyc to **/*.pyc.
• Add unit tests in flit_core/tests_core/test_sdist.py verifying that recursive glob patterns properly match and exclude dotfiles in FilePatterns and SdistBuilder.