BUG: Fix crash on annotating something as typing.Callable

Probably related: https://bugs.python.org/issue42195

	Traceback:
	...
	  File "~/pdoc/pdoc/__init__.py", line 1296, in _formatannotation
		return str(inspect.formatannotation(maybe_replace_reprs(annot)))
	  File "python3.7/inspect.py", line 1194, in formatannotation
		return repr(annotation).replace('typing.', '')
	  File "python3.7/typing.py", line 648, in __repr__
		return (f'typing.Callable'
	IndexError: tuple index out of range

pdoc/__init__.py

@@ -1285,8 +1285,11 @@ def maybe_replace_reprs(a):
         # nptyping.types._ndarray.NDArray -> NDArray[(Any,), Int[64]]  # GH-231
         if module.startswith('nptyping.'):
             return force_repr(repr(a))
-        # Recurse into args
+        # Recurse into typing.Callable/etc. args
         if hasattr(a, 'copy_with') and hasattr(a, '__args__'):
+            if a is typing.Callable:
+                # Bug on Python < 3.9, https://bugs.python.org/issue42195
+                return a
             a = a.copy_with(tuple([maybe_replace_reprs(arg) for arg in a.__args__]))
         return a
 

pdoc/test/__init__.py

@@ -902,6 +902,14 @@ def bug253_newtype_annotation(a: CustomType):
             pdoc.Function('bug253', mod, bug253_newtype_annotation).params(annotate=True),
             ['a:\N{NBSP}CustomType'])
 
+        # typing.Callable bug
+        def f(a: typing.Callable):
+            return
+
+        self.assertEqual(
+            pdoc.Function('f', mod, f).params(annotate=True),
+            ['a:\N{NBSP}Callable'])
+
         # builtin callables with signatures in docstrings
         from itertools import repeat
         self.assertEqual(pdoc.Function('repeat', mod, repeat).params(), ['object', 'times'])