lint: Add black as a linter.

tools/lint

@@ -1,6 +1,7 @@
 #! /usr/bin/env python3
 
 import argparse
+import re
 import sys
 
 from zulint.command import LinterConfig, add_default_linter_arguments
@@ -44,6 +45,15 @@ def run() -> None:
         description="Sorts Python import statements",
         check_arg=["--check-only", "--diff"],
     )
+    linter_config.external_linter(
+        "black",
+        ["black"],
+        ["py"],
+        description="Reformats Python code",
+        check_arg=["--check"],
+        suppress_line=lambda line: line == "All done! ✨ 🍰 ✨\n"
+        or re.fullmatch(r"\d+ files? would be left unchanged\.\n", line) is not None,
+    )
 
     @linter_config.lint
     def custom_py() -> int: