set pytest rootdir

With pytest 6.1.0 the root directory calculation logic has
changed and is unreliable on our jenkins setup.   make
sure this is set explicitly.

Change-Id: I1d48b887030c32d2f3361fc36e65e8a988d2ba81
References: pytest-dev/pytest#7807
(cherry picked from commit 7193f94294718159eeabc296812519b30dad67b5)

tox.ini

@@ -15,7 +15,7 @@ install_command=python -m pip install {env:TOX_PIP_OPTS:} {opts} {packages}
 usedevelop=
      cov: True
 
-deps=pytest!=3.9.1,!=3.9.2
+deps=pytest>4.6  # this can be 6.x once we are on python 3 only
      pytest-xdist
      mock; python_version < '3.3'
      # needed only for correct profiling results
@@ -56,7 +56,7 @@ setenv=
     PYTHONPATH=
     PYTHONNOUSERSITE=1
     MEMUSAGE=--nomemory
-    BASECOMMAND=python -m pytest --log-info=sqlalchemy.testing
+    BASECOMMAND=python -m pytest --rootdir {toxinidir} --log-info=sqlalchemy.testing
 
     WORKERS={env:TOX_WORKERS:-n4  --max-worker-restart=5}
     oracle: WORKERS={env:TOX_WORKERS:-n2  --max-worker-restart=5}