Remove unused imports from period.pyx

[jbrockmendel]

Replace isinstance checks with more efficient versions from util;

Remove redundant checks for isinstance(..., (timedelta, Timedelta)) since
Timedelta subclasses timedelta

The main goal is to reduce the inter-dependencies between lib/tslib/period(/frequencies/offsets).

[jreback]

an asv run for period benchmarks is useful for proving claims here. a simple posted timeit on performance claim is also useful to the causal reader.

[jbrockmendel]

I've been instructed to use the util versions wherever possible.

No idea what the Travis error means, so am inclined to drop it. This for now. asv will be easier if/when #17331 is merged.