setup: Dataclasses only when < 3.7 (#45844)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45844

Someone pointed out that dataclasses were actually added to the python
stdlib in 3.7 and not 3.8, so bumping down the dependency on dataclasses
from 3.8 -> 3.7 makes sense here

Signed-off-by: Eli Uriegas <eliuriegas@fb.com>

Test Plan: Imported from OSS

Reviewed By: walterddr, malfet

Differential Revision: D24113367

Pulled By: seemethere

fbshipit-source-id: 03d2d93f7d966d48a30a8e2545fd07dfe63b4fb3
diff --git a/setup.py b/setup.py
index b2270db..c29ee92 100644
--- a/setup.py
+++ b/setup.py
@@ -346,7 +346,7 @@
 install_requires = [
     'future',
     'typing_extensions',
-    'dataclasses; python_version < "3.8"'
+    'dataclasses; python_version < "3.7"'
 ]
 
 missing_pydep = '''