Namespace removal
There are a few places where methods/attributes have crept into class namespaces that probably don't belong there.  Will there be any objections to cleaning up any of the following?

`tslib.Timedelta._validate_ops_compat` does not use `self`, can be a `cdef` function at the module level.

`tslib.Timedelta._binary_op_method_timedeltalike` doesn't even have `self` in its args, can be a func at the module level (though it cant be cdef)

`tslib.Timedelta._op_unary_method` ditto

`tslib.Timedelta._round` and `tslib.Timestamp._round` are backend methods that `round, floor, ceil` call.  They could be `cdef` and not exposed.

`_Timestamp._get_field` is `cpdef`, could be `cdef` and not exposed.
`_Timestamp._get_start_end_field` ditto

All of these _technically_ break backward compat if there is someone out there using them, but I have a hard time imagining that use case, so think its worth considering.


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Outside of `tslib`, the items in my notes are:

`core.groupby.SeriesGroupBy._agg_doc`
`core.groupby.DataFrameGroupBy._agg_doc`
`DataFrame._agg_doc`
