tests: allow prerelease dependency versions under Python 3.9 (#129)

Also, remove 'tests/unit/test_exceptions.py' fossil.  The
'google.cloud.exceptions' module no longer contains any code, but is only
a set of backward-compatibility imports.

Author: tseaver

testing/constraints-3.9.txt

@@ -0,0 +1,2 @@
+# Allow prerelease requirements
+--pre