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/

packages/google-cloud-vision/setup.py

@@ -37,7 +37,7 @@
  author='Google Cloud Platform',
  author_email='googleapis-publisher@google.com',
  name='google-cloud-vision',
- version='0.28.0',
+ version='0.28.1.dev1',
  description='Python Client for Google Cloud Vision',
  long_description=readme,
  namespace_packages=[