Conditional requirement for py3.6 only (#46932)

Summary:
Fixes https://github.com/pytorch/pytorch/issues/46930

Pull Request resolved: https://github.com/pytorch/pytorch/pull/46932

Reviewed By: mrshenli

Differential Revision: D24574196

Pulled By: seemethere

fbshipit-source-id: 11daf8abe226670277f1b5682fd9890d23576271
diff --git a/requirements.txt b/requirements.txt
index 07127f7..759baf3 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -5,4 +5,4 @@
 setuptools
 six
 typing_extensions
-dataclasses
+dataclasses; python_version<"3.7"