feat: Ex command `:py=` evaluate and print python expression

[wookayin]

The Ex command :py, :python, :py3, etc. can evaluate the line as an expression rather than a statement if the line starts with =, just like :lua=.

:py= <expr> is equivalent as :py print(<expr>).

:py3= sys.version_info[:3]
:python3 =pynvim.__version__

TODO

• How to document into /doc/if_pyth.txt (neovim core)? The feature would require pynvim 0.5.0 (or devel). It would make sense to submit a PR to neovim core only after a stable release of pynvim that contains this patch is released.

Developer note:

should be merged (or rebased) after #547 to avoid conflicts (Merged)

[justinmk]

How to document into /doc/if_pyth.txt (neovim core)? The feature would require pynvim 0.5.0 (or devel).

Better to get it in there asap. One more motivation for doing a pynvim release :)

[wookayin]

Hmm now I think we may want to use repr(...) to print. For example, python3= "foo" might print 'foo' instead of foo only. Will think about it in any subsequent PRs....

Need to make the behavior consistent with https://github.com/neovim/neovim/issues/25379