Fix mypyc KeyError on src/black/parsing.py

Typing the ast3 / ast27 variables as Any breaks mypyc. More details in
the comment added in this commit.

Many thanks goes to Jelle for helping out, with their insight I was
able to find a workaround.

pyproject.toml

@@ -32,3 +32,6 @@ optional-tests = [
   "no_python2: run when `python2` extra NOT installed",
   "no_blackd: run when `d` extra NOT installed",
 ]
+markers = [
+  "incompatible_with_mypyc: run when testing mypyc compiled black"
+]

src/black/parsing.py

@@ -5,6 +5,11 @@
 import sys
 from typing import Any, Iterable, Iterator, List, Set, Tuple, Type, Union
 
+if sys.version_info < (3, 8):
+    from typing_extensions import Final
+else:
+    from typing import Final
+
 # lib2to3 fork
 from blib2to3.pytree import Node, Leaf
 from blib2to3 import pygram, pytree
@@ -134,6 +139,10 @@ def parse_ast(src: str) -> Union[ast.AST, ast3.AST, ast27.AST]:
     return ast27.parse(src)
 
 
+ast3_AST: Final[Type] = ast3.AST
+ast27_AST: Final[Type] = ast27.AST
+
+
 def stringify_ast(
     node: Union[ast.AST, ast3.AST, ast27.AST], depth: int = 0
 ) -> Iterator[str]:
@@ -173,7 +182,13 @@ def stringify_ast(
                 elif isinstance(item, (ast.AST, ast3.AST, ast27.AST)):
                     yield from stringify_ast(item, depth + 2)
 
-        elif isinstance(value, (ast.AST, ast3.AST, ast27.AST)):
+        # Note that we are referencing the typed-ast ASTs via global variables and not
+        # direct module attribute accesses because that breaks mypyc. It's probably
+        # something to do with the ast3 / ast27 variables being marked as Any leading
+        # mypy to think this branch is always taken, leaving the rest of the code
+        # unanalyzed. Tighting up the types for the typed-ast AST types avoids the
+        # mypyc crash.
+        elif isinstance(value, (ast.AST, ast3_AST, ast27_AST)):
             yield from stringify_ast(value, depth + 2)
 
         else:
@@ -192,7 +207,7 @@ def stringify_ast(
                 # To normalize, we strip any leading and trailing space from
                 # each line...
                 stripped = [line.strip() for line in value.splitlines()]
-                normalized = lineend.join(stripped)
+                normalized = lineend.join(stripped)  # type: ignore[attr-defined]
                 # ...and remove any blank lines at the beginning and end of
                 # the whole string
                 normalized = normalized.strip()