Fix automatic installation of dependencies.

[frantp]

Currently, setup.py imports the __init__.py file to get the package version; this prevents the automatic installation of pyserial, giving the following error if it is not manually installed before pypozyx:

Traceback (most recent call last):
    ...
    from serial import Serial, VERSION as PYSERIAL_VERSION, SerialException, line 10, in <module>
ModuleNotFoundError: No module named 'serial'

This patch moves the definition of the version string to a separate file, avoiding dependency imports during installation.