chore(gitignore): stop ignoring the repository's own lib/

The Python section of the .gitignore template has a bare 'lib/' (and
'lib64/'), which also matched the Nix helper modules in lib/. The
'!lib/*.nix' line added earlier could not help, since a file cannot be
re-included while its parent directory is excluded.

.gitignore

@@ -29,8 +29,6 @@ dist/
 downloads/
 eggs/
 .eggs/
-lib/
-lib64/
 parts/
 sdist/
 var/
@@ -229,6 +227,3 @@ __marimo__/
 
 # Streamlit
 .streamlit/secrets.toml
-
-# the library modules under lib/ must stay tracked
-!lib/*.nix