Changing all pypi.python.org links to warehouse links. (#3641)

Done via

$ export OLD='https\:\/\/pypi.python.org\/pypi\/'
$ export NEW='https\:\/\/pypi.org\/project\/'
$ git grep -l ${OLD} | xargs sed -i s/${OLD}/${NEW}/g

Then manually going through and adding a trailing slash to all
warehouse links.

(Though I did undo changes to `docs/json/`.)

Author: dhermes

README.rst

@@ -47,6 +47,6 @@ See the ``google-cloud-python`` API Error Reporting `Documentation`_ to learn
 how to get started using this library.
 
 .. |pypi| image:: https://img.shields.io/pypi/v/google-cloud-error-reporting.svg
-   :target: https://pypi.python.org/pypi/google-cloud-error-reporting
+   :target: https://pypi.org/project/google-cloud-error-reporting/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-error-reporting.svg
-   :target: https://pypi.python.org/pypi/google-cloud-error-reporting
+   :target: https://pypi.org/project/google-cloud-error-reporting/