Use maktaba#python#Eval for Python 3 support

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[blueyed]

@googlebot I signed it!

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CLAs look good, thanks!

[malcolmr]

You'll want to change .travis.yml and plugin/coverage.vim to update the minimum Maktaba version to at least 1.12.0 (was 1.9.0), since that's the version that added maktaba#python#Eval.

(1.14.0 was the version that added python3 support to maktaba#python#, but I don't think we need to explicitly test that version in addition - we're already testing against Maktaba master.)

[blueyed]

@malcolmr Thanks. What about the comment there? (https://github.com/google/vim-coverage/blob/97ad576f3c29302fb9d9def698f3ebc50f6a3413/.travis.yml#L4)

Ah, seems to have moved to plugin/coverage.vim.

[blueyed]

@malcolmr Thanks! Please squash-merge it then - or should I force-push already? (might save a build then, so I will do it)