CLN/DEPR: remove engine='python' from to_csv

[jreback]

the engine kw and older code: https://github.com/pydata/pandas/blob/master/pandas/core/format.py#L1364

let's deprecate in 0.17.1 and just remove in 0.18.0

This was never advertised / doc-ed so prob safe to remove

[taeold]

What does deprecation mean compared to removal? Are you looking for code emitting warning messages indicating that engine=python is no longer supported sufficient?

[jreback]

@taeold exactly what I mean, IOW, if engine=='python' (or actually is not None), then we emit a Future Warning that we are going to remove it in a future version. Then at some future release we will.

[jreback]

closed by #11319