Use gross hacks in ClassVar detection to avoid importing typing

Father forgive me for I have sinned.

ref ericvsmith/dataclasses#61

src/attr/_make.py

@@ -191,16 +191,14 @@ class MyClassAttributes(tuple):
 ])
 
 
-try:
- from typing import ClassVar
-
- # You cannot use ClassVar together with isinstance.
- _CLASS_VAR_CLS = ClassVar.__class__
-except ImportError:
- class FakeClassVar(object):
- pass
+def _is_class_var(annot):
+ """
+ Check whether *annot* is a typing.ClassVar.
 
- _CLASS_VAR_CLS = FakeClassVar
+ The implementation is gross but importing `typing` is slow and there are
+ discussions to remove it from the stdlib alltogether.
+ """
+ return str(annot).startswith("typing.ClassVar")
 
 
 def _transform_attrs(cls, these, auto_attribs):
@@ -231,7 +229,7 @@ def _transform_attrs(cls, these, auto_attribs):
  ca_list = []
  annot_names = set()
  for attr_name, type in anns.items():
- if isinstance(type, _CLASS_VAR_CLS):
+ if _is_class_var(type):
  continue
  annot_names.add(attr_name)
  a = cd.get(attr_name, NOTHING)