fix mypy by correcting the __lt__ annotation

pre_commit_hooks/requirements_txt_fixer.py

diff --git a/pre_commit_hooks/requirements_txt_fixer.py b/pre_commit_hooks/requirements_txt_fixer.py
index 78103a1..2412633 100644
--- a/pre_commit_hooks/requirements_txt_fixer.py
+++ b/pre_commit_hooks/requirements_txt_fixer.py
@@ -36,7 +36,7 @@
 
         return name[:m.start()]
 
-    def __lt__(self, requirement: 'Requirement') -> int:
+    def __lt__(self, requirement: 'Requirement') -> bool:
         # \n means top of file comment, so always return True,
         # otherwise just do a string comparison with value.
         assert self.value is not None, self.value