don't require typed-ast

setup.py

@@ -71,7 +71,7 @@ def get_long_description() -> str:
         "click>=7.1.2",
         "appdirs",
         "toml>=0.10.1",
-        "typed-ast>=1.4.2",
+        "typed-ast>=1.4.2; python_version < '3.8'",
         "regex>=2020.1.8",
         "pathspec>=0.6, <1",
         "dataclasses>=0.6; python_version < '3.7'",
@@ -81,6 +81,7 @@ def get_long_description() -> str:
     extras_require={
         "d": ["aiohttp>=3.3.2", "aiohttp-cors"],
         "colorama": ["colorama>=0.4.3"],
+        'python2': ['typed-ast>=1.4.2'],
     },
     test_suite="tests.test_black",
     classifiers=[

src/black/__init__.py

@@ -48,7 +48,20 @@
 from dataclasses import dataclass, field, replace
 import click
 import toml
-from typed_ast import ast3, ast27
+
+if sys.version_info < (3, 8):
+    try:
+        from typed_ast import ast3, ast27
+    except ImportError:
+        print(
+            "The typed_ast package is not installed.\n"
+            "You must install typed_ast with `python3 -m pip install typed-ast`.",
+            file=sys.stderr,
+        )
+        sys.exit(1)
+else:
+    ast3 = ast27 = ast
+
 from pathspec import PathSpec
 
 # lib2to3 fork