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

[basnijholt]

As by the suggestion of @Carreau in #179.

[akhmerov]

I see the following in py27:

pipinstallgit+https://github.com/python-adaptive/adaptive.git@python_requires (py27) Collectinggit+https://github.com/python-adaptive/adaptive.git@python_requiresCloninghttps://github.com/python-adaptive/adaptive.git (torevisionpython_requires) to/tmp/pip-req-build-uRyzYICompleteoutputfromcommandpythonsetup.pyegg_info:
Traceback (mostrecentcalllast):
File"<string>", line1, in<module>File"/tmp/pip-req-build-uRyzYI/setup.py", line19, in<module>version, cmdclass=get_version_and_cmdclass('adaptive')
File"/tmp/pip-req-build-uRyzYI/setup.py", line11, inget_version_and_cmdclassfromimportlib.utilimportmodule_from_spec, spec_from_file_locationImportError: Nomodulenamedutil----------------------------------------Command"python setup.py egg_info"failedwitherrorcode1in/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.