Update mypy 0.720 -> 0.740

Changelogs:
http://mypy-lang.blogspot.com/2019/09/mypy-730-released.html
http://mypy-lang.blogspot.com/2019/10/mypy-0740-released.html

New errors:
src/_pytest/recwarn.py:77: error: Missing return statement
src/_pytest/recwarn.py:185: error: "bool" is invalid as return type for "__exit__" that always returns False
src/_pytest/recwarn.py:185: note: Use "typing_extensions.Literal[False]" as the return type or change it to "None"
src/_pytest/recwarn.py:185: note: If return type of "__exit__" implies that it may return True, the context manager may swallow exceptions
src/_pytest/recwarn.py:185: error: Return type "bool" of "__exit__" incompatible with return type "None" in supertype "catch_warnings"
src/_pytest/recwarn.py:230: error: "bool" is invalid as return type for "__exit__" that always returns False
src/_pytest/recwarn.py:230: note: Use "typing_extensions.Literal[False]" as the return type or change it to "None"
src/_pytest/recwarn.py:230: note: If return type of "__exit__" implies that it may return True, the context manager may swallow exceptions
src/_pytest/recwarn.py:230: error: Return type "bool" of "__exit__" incompatible with return type "None" in supertype "catch_warnings"

The errors are due to this new error:
https://mypy.readthedocs.io/en/latest/error_code_list.html#check-the-return-type-of-exit-exit-return

.pre-commit-config.yaml

@@ -42,7 +42,7 @@ repos:
     hooks:
     -   id: rst-backticks
 -   repo: https://github.com/pre-commit/mirrors-mypy
-    rev: v0.720
+    rev: v0.740
     hooks:
     -   id: mypy
         files: ^(src/|testing/)