Use default utf-8 encoding for string decode() and encode()

[Eric-Arellano]

Python 3 defaults to using utf-8 for these two methods: https://docs.python.org/3/library/stdtypes.html#str.encode.

By using the default encoding, we result in less code and less cognitive overload, because the default works and shouldn't require thinking about it. We should only specify the encoding when it's not utf-8, like ascii.

This change was generated via this script.

[Eric-Arellano]

The second commit (d6640b8) is autogenerated, first and third are manual (8c4b92a and 980eba4).