Update .gitignore (#1678)

This PR updates .gitignore, pruning some unnecessary bits and pieces and making some existing entries stricter.

For the .mypy_cache removal, note that a .mypy_cache directory already has its own .gitignore file, so there's no need to mention it in the repo-level .gitignore. Similarly, the htmlcov directory created by coverage html already includes its own .gitignore.

.gitignore

@@ -1,20 +1,18 @@
-.coverage.*
-# file types to ignore
+# Compiled Python bytecode
 *.pyc
+
+# Compiled C extensions
 *.pyd
 *.so
-*.enamlc
-*~
-.DS_Store
 
-# ignore the build directories
+# Build directories
 *.egg-info/
-build/
-dist/
-docs/build/
+/build/
+/dist/
+/docs/build/
 
-# ignore mypy cache
-.mypy_cache/
+# Coverage artifacts
+/.coverage
 
-# Auto-generated by setup.py
-traits/version.py
+# Version file (auto-generated by setup.py)
+/traits/version.py