Namespace removal

[jbrockmendel]

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

[jbrockmendel]

_Period._comparables appears unused.

[jbrockmendel]

Added figuring these out to the TODO list in #17652. closing.