fix: align Python version floor with KiCad's macOS bundle (3.9)

KiCad ships Python 3.9 on macOS (both v9 and v10), and `pcbnew.so` is
compiled against that interpreter. The previous `>=3.10` floor was
unreachable on macOS and docs referenced nonexistent
`python3.11/site-packages` paths. Lower the floor and correct the docs;
no source changes needed — all 49 files in `python/` already compile
clean on 3.9.

Also add `.venv/` to `.gitignore` (dotted form used by `python -m venv`).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -39,6 +39,7 @@ dmypy.json
 
 # Virtual Environments
 venv/
+.venv/
 env/
 ENV/