This option simplifies the configuration options:
1) `g:python{,3}_host_prog` is not set.
Neovim tries its best to find a suitable interpreter. This means calling
exepath(), potentially multiple times, and a system('python -c ...') with
the first found interpreter, to get the Python version.
2) `g:python{,3}_host_prog` is set.
Avoids everything of the above. No safety checks, no training wheels. Fast
host startup time!
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