try picking up installed package with --pyargs

doc/sphinx/guide/extending.rst

@@ -25,7 +25,7 @@ The simplest solution is to load it directly when your package is imported.
 In the current example, this could be done by adding the following
 line to the end of the file::
 
-    init(periodictable.core.elements)
+    init(periodictable.core.default_table())
 
 This would be fine for the current example because the table size is
 small and load time is fast.  For large tables, you may wish to

pytest.ini

@@ -1,6 +1,7 @@
 [pytest]
 addopts = --doctest-modules --doctest-glob=*.rst --cov=periodictable
 doctest_optionflags = ELLIPSIS
+pythonpath = doc/sphinx
 testpaths = periodictable test doc/sphinx/guide
 python_files = *.py
 python_classes = NoClassTestsWillMatch