bpo-31361 : Update feedparser.py to prevent theano compiling fail in python3

[ralic]

When data is None, the installation of theano got crash.
Add str() wrapper to the data for file writing.

https://bugs.python.org/issue31361

[the-knights-who-say-ni]

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[bitdancer]

Please open an issue on bugs.python.org with instructions on how to reproduce the problem without involving theano, and/or explain what it is that theano is doing when the problem occurs.

[merwok]

The bug is in pip.