Marking all remaining versions as "dev". (#4299)

This is to make it clear the code is between releases. Any code
that relies on a **new** feature (e.g. of `google-api-core`) will
then be able to **explicitly** make this clear by using the
lower bound of the `devN` version.

Fixes #4208.

See: https://snarky.ca/how-i-manage-package-version-numbers/

Author: dhermes

setup.py

@@ -59,7 +59,7 @@
 
 setup(
     name='google-cloud-error-reporting',
-    version='0.28.0',
+    version='0.28.1.dev1',
     description='Python Client for Stackdriver Error Reporting',
     long_description=README,
     namespace_packages=[