Do not use `finish` inside of `provider#python{,3}#Call`, but `return`.

"python -c" returns 1 in case of an error. Use a return code of 2 if
the Neovim module is not found to distinguish these cases.

Verify the interpreter version before checking for an installed Neovim
module. Show a new error message if the Python interpreter version
is below the minimum required version.

Always use "pkgutil" to determine if the Neovim module is installed.
In contrast to "importlib", which was used for Python 3,
"pkgutil.find_loader" is available for all Python versions [1,2].
"pkgutil.find_loader" internally uses "importlib" for Python >= 3.3 [2].
Also, the previously used "importlib.find_loader" is only available
since Python 3.3 (so checking the major Python version was not enough)
and deprecated since Python 3.4 [3].
Finally, conditioning on the major version in Vimscript was incorrect,
as checking the Neovim module for a certain Python major version does
not mean that the tested interpreters are actually of that version.
For example, we test the "python" executable, which is Python 2 on
Ubuntu and Python 3 on Arch Linux.

[1] https://docs.python.org/2/library/pkgutil.html#pkgutil.find_loader
[2] https://docs.python.org/3/library/pkgutil.html#pkgutil.find_loader
[3] https://docs.python.org/3/library/importlib.html#importlib.find_loader

Before, running Nvim in a directory containing a Python module `neovim`,
or one that is imported by it or a plugin, will load that module and not
the system one. So Nvim might be tricked into running arbitrary scripts
from the current working directory.

Fixes #1665
Fixes #2530

Python: fixes for sys.path_hooks handler

DecodeHook isn't needed since neovim/python-client#53
rpc methods won't pass bytes anymore, ref neovim/python-client#129

This method was not used, but `VimModuleLoader.load_module` is.

The path hook used to load the module already in the `find_module` hook.

This caused different behaviour between Python 2.7 and 3.3, where the
former would call the `VimModuleLoader`, while Python 3.3 appears to
short-circuited this (because the module was loaded already).

This patch will now only find the module, but not load it in the
`find_module` hook.

This is required by PEP302 for `reload()`ing modules.