v0.28.0: Making release for most packages. (#4296)

Documentation

• Added link to "Python Development Environment Setup Guide" in
  project README (#4187, h/t to @michaelawyu)

Dependencies

• Upgrading to google-cloud-core >= 0.28.0 and adding dependency
  on google-api-core (#4221, #4280
• Upgrading to google-cloud-logging >= 1.4.0 (#4296)

PyPI: https://pypi.org/project/google-cloud-error-reporting/0.28.0/