BREAKING CHANGE: A config file is no longer needed to run
the CLI. python-gitlab will default to https://gitlab.com
with no authentication if there is no config file provided.
python-gitlab will now also only look for configuration
in the provided PYTHON_GITLAB_CFG path, instead of merging
it with user- and system-wide config files. If the
environment variable is defined and the file cannot be
opened, python-gitlab will now explicitly fail.

BREAKING CHANGE: python-gitlab will now default to gitlab.com
if no URL is given

Before we raised an error if there was a 301, 302 redirect but only
from an http URL to an https URL.  But we didn't raise an error for
any other redirects.

This caused two problems:

  1. PUT requests that are redirected get changed to GET requests
     which don't perform the desired action but raise no error. This
     is because the GET response succeeds but since it wasn't a PUT it
     doesn't update. See issue:
     https://github.com/python-gitlab/python-gitlab/issues/1432
  2. POST requests that are redirected also got changed to GET
     requests. They also caused hard to debug tracebacks for the user.
     See issue:
     https://github.com/python-gitlab/python-gitlab/issues/1477

Correct this by always raising a RedirectError exception and improve
the exception message to let them know what was redirected.

Closes: #1485
Closes: #1432
Closes: #1477

This is useful when docs aren't included in the toctree and don't show up on RTD.