`RedirectStream` is used to redirect `stdout` and `stderr`, but are
missing certain I/O methods available on other file-like objects.
This causes external plugins (like `colorama`) to crash.

Inheriting from `io.IOBase` adds an abstract implementation of these
methods, which will at least keep the python code running.

Fixes #4045

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