gracefully handle if git is not installed

The following error occurs in `_get_contributors()` if git is not
installed.

> FileNotFoundError: [Errno 2] No such file or directory: 'git': 'git'

Treat this the same as if `git` fails, catch the error and print a
warning.

sphinxcontrib/spelling/filters.py

@@ -245,7 +245,7 @@ def _get_contributors(self):
 
         try:
             p = subprocess.run(cmd, check=True, stdout=subprocess.PIPE)
-        except subprocess.CalledProcessError as err:
+        except (subprocess.CalledProcessError, FileNotFoundError) as err:
             logger.warning('Called: %s', ' '.join(cmd))
             logger.warning('Failed to scan contributors: %s', err)
             return set()