Detect python version

[michielboekhoff]

This PR allows users to manually specify the Python interpreter to use when installing Poetry. If one is not passed, it will try to find a python3 executable on the PATH, and otherwise default to the python executable. This is an implementation as discussed with @brycedrennan in #1042.

[michielboekhoff]

Not sure what's going on with the Windows builds there.

The Linux 2.7 build appears to be failing as it was using the python3 executable. I'm fixing this in the CI now.

[Jamim]

Similar PR: #1494.

[michielboekhoff]

@Jamim - thanks for that!

This is as per the discussion in #1042, going a few steps further than what was done there and in #1494.

[nottrobin]

Can't wait for this! When I saw https://github.com/python-poetry/poetry/releases/tag/1.0.0 had been released, I really thought it would have included this fix :(

Please let me know if there's any way I could help.

[michielboekhoff]

@nottrobin - cheers for the enthusiasm about this! I've just updated this project against the latest develop branch.

Apologies about the force-pushing; I did this to clean up some commits and to keep the history clean.

If anyone would like to take a look at this, it'd be much appreciated. I'd still love to see this included in Poetry.

[nottrobin]

A note: When running poetry v1.0.0 I'm still getting a pip not found error (in Ubuntu 18.04 when I don't have python2 installed & haven't manually created a symlink). I thought this had been fixed in previous work, but it must have crept back in.

Should this PR use a similar mechanism for ensuring that the pip binary used is appropriate? Either {python} -m pip {command} or {pip} {command}?

[mvoitko]

This probably should not be None

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