Prefer _version.py over importlib.metadata at import time

[jstepgoogle]

Try importing the build-time generated tabulate._version module first, and only import importlib.metadata as a fallback when _version.py is not present (e.g. in an unbuilt git checkout).

Since pyproject.toml configures flit_scm / setuptools_scm with write_to = "tabulate/_version.py", standard wheels and sdists already include _version.py. Importing it directly avoids loading importlib.metadata (and its transitive zipfile, email, pathlib, and csv imports) and scanning sys.path for .dist-info directories on every import tabulate.