Is `@td.skip_if_no("pathlib")` necessary?

Admittely this is very minor, but 
there's a few parts in the codebase (e.g. `pandas/tests/io/test_common.py`) where there is
```
@td.skip_if_no("pathlib")
```
As of `v1.0`, the minimum Python version supported will be 3.6.1, so `pathlib` will always be present as part of the standard library.