Revert "tests/api/graph: Ensure tests pass with all requests-mock versions"

This reverts commit 50c6b1f01fb6de10498a89e37e901382b773eeeb.

The issue was coming from a bug in requests-mock that is now fixed
in version 1.9.2 of the package so revert introduced workaround
and exclude buggy versions of requests-mock in requirements-test.

See https://github.com/jamielennox/requests-mock/issues/170