Remove sitecustomize file (spotify#1755)

See discussion in spotify#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.

Author: Tarrasch

.coveragerc

@@ -10,8 +10,8 @@ omit =
     */.tox/*
     */setup.py
     */bin/luigidc
-    sitecustomize.py
     hadoop_test.py
     minicluster.py
 [run]
 parallel=True
+concurrency=multiprocessing

sitecustomize.py

@@ -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>=4.1,<4.2
   codecov>=1.4.0
   requests<3.0
   unixsocket: requests-unixsocket<1.0