Fix error when using is_version with lettered tmux version

[minijackson]

is_version('something') fails with installed tmux 1.9a

To allow tmux-python/tmuxp#197 ;-)

[codecov-io]

Current coverage is 78.74% (diff: 100%)

Merging #22 into master will increase coverage by 0.49%

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[tony]

@minijackson Can this have a test for it please?

[tony]

Also look at this: https://stackoverflow.com/questions/11887762/compare-version-strings/21065570#21065570

[minijackson]

There you go!

But I don't know how to mock the installed tmux to be tmux 1.9a, so while the case that the requested tmux version is "lettered" is tested, the installed tmux case is not.

[tony]

Travis automatically tests across those tmux versions. https://github.com/tony/libtmux/blob/master/.travis.yml

also you could manually insert the version number for '1.9a'

[minijackson]

also you could manually insert the version number for '1.9a'

I didn't understand, is it something I have to do for this PR ? Where should I insert the version number ?

[tony]

That is fine.

The only thing I'm double checking is how broad support is for pkg_resources. Are there any corner cases where the user may not have setuptools and this patch could lead to a ImportError: No module named 'pkg_resources'.

[tony]

Even though most machines are going to have setuptools on it, after further consideration it may be safer to use a method without it. I'm not sure what the ramifications would be for upstream packages for tmuxp on arch and slack.

[tony]

Try LooseVersion instead of StrictVersion.