Fix: evaluate v:true/v:false to python bool type

[uzxmx]

This PR fixes an issue that when calling vim.eval('v:true'), or vim.eval('v:false'), it gives the wrong str type.

In the latest version of vim, when executing below Ex command, the result is <class 'bool'>. But in neovim, it is <class 'str'>.

:py3 print(type(vim.eval('v:true')))

[wookayin]

Follow-up: There was a concern raised in #523 (comment) that it doesn't conform with the python-eval doc: https://neovim.io/doc/user/if_pyth.html#python-eval

[justinmk]

Can you send a PR to update the docs?

[wookayin]

I think more decisions on the behavior of python-eval would be needed. Same as in the legacy vim, do we want to make the return type as string for numeric/boolean values? What's the main rationale behind any decision?

@uzxmx Do you have any idea why you wanted this change?

[justinmk]

Reverted in #562

See #523 (comment) :

the #506 PR is not compliant with the vim.eval documentation.

The documentation states

vim.eval(str) python-eval
Evaluates the expression str using the vim internal expression
evaluator (see expression). Returns the expression result as:
(1) a string if the Vim expression evaluates to a string or number
(2) a list if the Vim expression evaluates to a Vim list
(3) a dictionary if the Vim expression evaluates to a Vim dictionary Dictionaries and lists are recursively expanded.

In Python, booleans are implemented as a subclass of integers. In nvim, v:true and v:false evalutes to true and false (boolean values); for boolean values vim uses numbers. Therefore, by (1) and that booleans are integers in Python, vim.eval('v:true') should evaluate to the Python string "True" and not the boolean True. And therefore :py3 print(type(vim.eval('v:true'))) should print out <class 'str'> and not <class 'bool'>. Thus, the extra check in #506 is redundant, right?