Add single quotes around the password.

When the DB password uses some special characters, the
defaults-extra-file can be incorrect. In the case of a password with
the # symbol, anything after that is considered a comment. The single
quotes around the password rectify this.

borgmatic/hooks/data_source/mariadb.py

@@ -68,7 +68,7 @@ def make_defaults_file_options(username=None, password=None, defaults_extra_file
     values = '\n'.join(
         (
             (f'user={username}' if username is not None else ''),
-            (f'password={password}' if password is not None else ''),
+            (f'password=\'{password}\'' if password is not None else ''),
         )
     ).strip()