Do not use str() to generate RSS

[tobinus]

Currently, you can generate the RSS by converting the Podcast to RSS:

print(str(p))

However, this introduces a couple issues:

• Multiple ways of achieving the same thing.
• May accidentally cause the podcast RSS to be generated many times over, since users (rightly) think that str() should not be a costly operation.

For version 1.2.0, using str() to generate the RSS should yield DeprecationWarning.

For version 2.0.0, you can only use rss_str() to generate the RSS as string.