Building on Windows should remove all `__pycache__`

Feature or enhancement

The file Makefile.pre.in already has this target:

pycremoval:
	-find $(srcdir) -depth -name '__pycache__' -exec rm -rf {} ';'
	-find $(srcdir) -name '*.py[co]' -exec rm -f {} ';'

It would be useful to have an equivalent on Windows.

Pitch

This would be helpful when iterating on compiler/bytecode, instead of manually bumping the magic number at each build or manually removing all pycs. It's easy to forget and think there's a bug when in reality the .pyc files are just stale.

從 Makefile.pre.in 中現有的 pycremoval 目標開始，找出提供等效清理功能的 Windows 建置進入點。確保 Windows 工作流程刪除 pycache 目錄和 *.py[co] 檔案，然後驗證在編譯器或位元組碼反覆迭代期間可以使用該清理功能。