Move test dependencies into the package, removing 'tests' directory which...

Move test dependencies into the package, removing 'tests' directory which masks the error reported in #1896.

tox.ini

@@ -8,7 +8,7 @@
 envlist=python
 
 [testenv]
-deps=-rtests/requirements.txt
+deps=-rsetuptools/tests/requirements.txt
 # Changed from default (`python -m pip ...`)
 # to prevent the current working directory
 # from being added to `sys.path`.