Pin vendoring to ~=0.3.3 in test suite

Co-authored-by: Pradyun Gedam <3275593+pradyunsg@users.noreply.github.com>

tox.ini

@@ -70,7 +70,7 @@ basepython = python3
 skip_install = True
 commands_pre =
 deps =
-    vendoring>=0.3.3
+    vendoring~=0.3.3
     # Required, otherwise we interpret --no-binary :all: as
     # "do not build wheels", which fails for PEP 517 requirements
     pip>=19.3.1