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-trace/setup.py

@@ -15,7 +15,7 @@
 
 setup(
     name='google-cloud-trace',
-    version='0.16.0',
+    version='0.16.1.dev1',
     author='Google Inc',
     author_email='[email protected]',
     classifiers=[