TST: Update tox config to fix timeouts in CI.

Author: rossbar

tox.ini

@@ -11,6 +11,8 @@ description = run tests
 setenv =
     devdeps: PIP_EXTRA_INDEX_URL =  https://pypi.anaconda.org/scientific-python-nightly-wheels/simple
 
+passenv = OMP_NUM_THREADS
+
 deps =
     # We use these files to specify all the dependencies, and below we override
     # versions for specific testing schenarios