Attempt to add mac testing

This also failed for me with "future" being installed.

Not sure if it makes sense to keep it in the first place, since isort
appears to require py34 now.

Ref: https://github.com/timothycrosley/isort/issues/479#issuecomment-316235116

Restore `chdir` after tests.

This is relevant for when pytest-cov would display coverage in the end,
and would use absolute paths due to this.

Fix whitespace in warning message

Feature/fix windows support

Use settings_path instead of additional settings dict; remove encoding set as it didn't seem to fix the windows issue

Add LRU cache to RequirementsFinder._get_files_from_dir

This caches the result of the function for future invocations with the same
input, which can massively speed up future invocations if they have the same
input as previous calls.

In the case of this function this happens a lot, as it always is tried with all
parent paths in the file system from `isort` starting point.

The "backports.functools_lru_cache" package is used for Python 2 as it doesn't
have the built in `lru_cache` decorator which was added in Python 3.2.

Some benchmarks against a folder with 182 Python files with all finders enabled:

Python 2.7.15: Before 32 seconds, after 4 seconds.
Python 3.7.2: Before 28 seconds, after 2 seconds.

Disable requirements based finders