Fixed an incorrect version number for mypy in requirements3.txt

 - Legacy-Id: 16807

Author: levkowetz

requirements3.txt

@@ -36,7 +36,7 @@ jwcrypto>=0.4.0			# for signed notifications
 #lxml>=3.4.0                    # from PyQuery; 
 markdown2>=2.3.8
 mock>=2.0.0
-mypy>=0.7.3
+mypy==0.720
 mysqlclient>=1.3.13
 oauth2client>=4.0.0		# required by google-api-python-client, but not always pulled in
 patch>=1.16,<2.0