[4.6] tests: restore tracing function

Without this, `testing/test_pdb.py` (already without pexpect) will cause
missing test coverage afterwards (for the same process).

testing/conftest.py

@@ -1,6 +1,21 @@
 # -*- coding: utf-8 -*-
+import sys
+
 import pytest
 
+if sys.gettrace():
+
+    @pytest.fixture(autouse=True)
+    def restore_tracing():
+        """Restore tracing function (when run with Coverage.py).
+
+        https://bugs.python.org/issue37011
+        """
+        orig_trace = sys.gettrace()
+        yield
+        if sys.gettrace() != orig_trace:
+            sys.settrace(orig_trace)
+
 
 @pytest.hookimpl(hookwrapper=True, tryfirst=True)
 def pytest_collection_modifyitems(config, items):