Remove sitecustomize file

See discussion in #1693

Luigi developrs might need to run `rm .coverage` after applying this
change as a new version of coverage will be in use after this patch.

tox.ini

@@ -20,7 +20,7 @@ deps=
   cdh,hdp: hdfs>=2.0.4,<3.0.0
   postgres: psycopg2<3.0
   gcloud: google-api-python-client>=1.4.0,<2.0
-  coverage>=3.6,<3.999
+  coverage>=4.0,<5.0
   codecov>=1.4.0
   requests<3.0
   unixsocket: requests-unixsocket<1.0
@@ -47,7 +47,8 @@ setenv =
 commands =
   cdh,hdp: {toxinidir}/scripts/ci/setup_hadoop_env.sh
   python --version
-  coverage run test/runtests.py -v --ignore-files='(^\.|^_|^setup\.py$)' \
+  coverage run --concurrency=multiprocessing test/runtests.py -v
+                                   --ignore-files='(^\.|^_|^setup\.py$)' \
                                    --ignore-files=not_imported.py \
                                    {posargs:}
   coverage combine