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>

pyproject.toml

@@ -1,18 +1,18 @@
 [project]
 name = "kicad-mcp-server"
 version = "2.1.0"
-requires-python = ">=3.10"
+requires-python = ">=3.9"
 
 [tool.black]
 line-length = 100
-target-version = ["py310"]
+target-version = ["py39"]
 
 [tool.isort]
 profile = "black"
 line_length = 100
 
 [tool.mypy]
-python_version = "3.10"
+python_version = "3.9"
 warn_return_any = false
 warn_unused_configs = true
 check_untyped_defs = false