bindings: python: decouple the version of the bindings from libgpiod API version

bindings: python: decouple the version of the bindings from libgpiod API version

Python bindings now have their own setup.py script and can be built
separately from the rest of the code-base. Let's decouple the python
package version from libgpiod API (but let's keep a module attribute
containing the API version) by introducing a version.py submodule that
can be executed by the setup.py script. This way we have a single
canonical place defining the version number.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>