theme comparison: Print nice error message if GitPython is not found

theme_comparison.py

@@ -14,7 +14,6 @@
 import argparse
 from pathlib import Path
 
-import git
 from sphinx.cmd.build import build_main
 
 
@@ -36,6 +35,13 @@
     help='theme names (according to "*-theme" branch names)')
 args = parser.parse_args()
 
+try:
+    import git
+except ImportError as e:
+    parser.exit(
+        'The Python package GitPython has to be installed:\n\n'
+        '    python3 -m pip install GitPython\n')
+
 main_dir = Path(__file__).resolve().parent / 'theme_comparison'
 main_dir.mkdir(exist_ok=True)
 repo = git.Repo(main_dir, search_parent_directories=True)