Revert "TST: move conftest.py to top-level (pandas-dev#15731)"

This reverts commit 6a52c15.

Author: jreback

conftest.py renamed to pandas/conftest.py

@@ -29,7 +29,7 @@ class TestPDApi(Base, tm.TestCase):
 
     # these are optionally imported based on testing
     # & need to be ignored
-    ignored = ['tests', 'locale']
+    ignored = ['tests', 'locale', 'conftest']
 
     # top-level sub-packages
     lib = ['api', 'compat', 'computation', 'core',