Change helpers to use pathlib.Path

[abravalheri]

In the past when introducing the extensions API, one point that was controversial was the usage of either lists of file path parts or strings using the forward slash as separator for helpers.ensure and helpers.reject.

It was suggested that we could use pathlib.Path to simplify the code.

Now that we run on Python 3 and pathlib is part of the standard library we can discuss more about that.

Since it breaks the API, if we go for this approach, we would have to start deprecating it now so we can include it in the next major release.

[abravalheri]

If we decide to adopt the new behaviour:

v3.2 - Deprecation Notice
v4.0 - Actual change

[FlorianWilhelm]

@abravalheri Sounds like a great idea. Since it was included in Python 3.4 we should do it as you said. Thanks!

[abravalheri]

Implemented in #215 with deprecation notices.
TODO note added explaining how to drop support for lists/tuples in v4.