CLN: cleanup libs cimports, remove is_timestamp
- cleanup imports in `_libs.parsers`, including some unused
- a couple of functions from util are used exactly once and only in parsers.  Get those directly from numpy_helper instead of from util
- remove unused `util.is_contiguous`
- remove a bunch of distutils shebang-like declarations that are unnecessary and are a pain for when we _do_ want to run tests with coverage enabled.
- remove `tslib.is_timestamp` (will be helpful for eventual goal of "fixing" Timestamp classmethods)
- remove some leftover constants from tslib, update imports appropriately