diff --git a/.gitignore b/.gitignore
index 494e06b..283c869 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,5 @@
 .coverage
 coverage.xml
-sitecustomize.py
 docs/_build
 *.egg-info
 *.egg
diff --git a/sitecustomize.py b/sitecustomize.py
new file mode 100644
index 0000000..9b5135d
--- /dev/null
+++ b/sitecustomize.py
@@ -0,0 +1,9 @@
+"""Setup coverage tracking for subprocesses.
+
+Any ImportError is silently ignored.
+Requires COVERAGE_PROCESS_START in the environments, which gets set in
+tox.ini.
+"""
+import coverage
+
+coverage.process_startup()