Misc cleanups, test parametrization
No coherent theme: remove an unused variable in inference.pyx, make some imports more direct, parametrize a big test, and move a tslib-specific test to test_tslibs
Hello @jbrockmendel! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on July 01, 2018 at 02:49 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21690?src=pr&el=h1) Report
> Merging [#21690](https://codecov.io/gh/pandas-dev/pandas/pull/21690?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/dc45fbafef172e357cb5decdeab22de67160f5b7?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21690/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21690?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21690   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         154      154           
  Lines       49657    49657           
=======================================
  Hits        45638    45638           
  Misses       4019     4019
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.28% <ø> (ø)` | :arrow_up: |
| #single | `41.9% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21690?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21690?src=pr&el=footer). Last update [dc45fba...2e4a2cd](https://codecov.io/gh/pandas-dev/pandas/pull/21690?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!