Document supported Python versions

[mjpieters]

Currently, there is no (easy to find) overview of what Python versions Click supports.

• The README is silent on the subject
• setup.py has trove identifiers for Programming Language :: Python and Programming Language :: Python :: 3, implying only Python 3 is supported.
• Although the distributed wheel on PyPI is built to be version agnostic, the Py Version column is set to the confusing value of 3.5 implying that the package only works on that Python version (*).
• The Python 3 page in the documentation decries Python 3 for how it handles text, and tells you to stick to Python 2 instead.
• The travis config shows the project is tested against 2.6, 2.7, 3.3 - 3.5 and pypy. This may or may not amount to the officially supported version list.

Please add, to the README and perhaps to the quickstart, a clear list of officially supported Python versions, and update the project trove classifiers to include, at least, Programming Language :: Python :: 2, or preferably all supported versions.

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(*) the Py Version column was taken from the Python version that built the wheel; use twine to upload wheels instead and that column will be set correctly.

[schlamar]

BTW, click fails with Python 2.6 on Windows (ref https://github.com/kennethreitz/requests/pull/3988).

[PyDever]

was this ever added?