use setup(..., python_requires='>=3.6'), closes #179

[basnijholt]

As by the suggestion of @Carreau in #179.

[akhmerov]

I see the following in py27:

pip install git+https://github.com/python-adaptive/adaptive.git@python_requires                                                                                                      (py27) 
Collecting git+https://github.com/python-adaptive/adaptive.git@python_requires
  Cloning https://github.com/python-adaptive/adaptive.git (to revision python_requires) to /tmp/pip-req-build-uRyzYI
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-req-build-uRyzYI/setup.py", line 19, in <module>
        version, cmdclass = get_version_and_cmdclass('adaptive')
      File "/tmp/pip-req-build-uRyzYI/setup.py", line 11, in get_version_and_cmdclass
        from importlib.util import module_from_spec, spec_from_file_location
    ImportError: No module named util
    
    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-req-build-uRyzYI/

[Carreau]

Both are not incompatible. This check is still useful if you have users with pip < 9.0.1 (which is still often the case for Python 2 users).

[basnijholt]

Thanks for checking, I've reverted that change.

[akhmerov]

LGTM. pip still fails on 2.7 if installing from source, but that's an unrelated issue which we should fix separately.