API Core: specify a pytype output directory in setup.cfg. (#7639)

The next version of pytype will change the default name of the pytype
output directory from pytype_output to .pytype, and `pytype_output/` is
in google-cloud-python's .gitignore. The easiest way to avoid being
affected by this change is to specify a custom output dir.
diff --git a/setup.cfg b/setup.cfg
index 973b943..581d862 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -7,3 +7,4 @@
     google/
 exclude =
     tests/
+output = pytype_output/