specify maximum versions for requests & urllib3 (#229)

Wouldn't be necessary if pip would do proper version management.
As things are right now, pip ignores the urllib3 version requirements
from requests because gallery-dl is specifying its own.

requirements.txt

@@ -1,2 +1,2 @@
-requests>=2.11.0
-urllib3>=1.16, !=1.24.1, !=1.24.2
+requests>=2.11.0, <2.22.0
+urllib3>=1.16, <1.25, !=1.24.1, !=1.24.2