python/CMakeLists.txt: Fix install destination

[podsvirov]

Added CMAKE_INSTALL_PYLIBDIR cached variable. Default value retrivet from Python3_EXECUTABLE.

Current install destination Python3_SITELIB (absolute path from FindPython3 module). An absolute path is not acceptable when creating an installation package.

[jmr]

Does #186 make this unnecessary?

[podsvirov]

These changes are not directly related to the #186. These changes solve the problem of hard-coded absolute installation path for the python module. Absolute install paths add additional problems when creating packages for distribution (in this case, it becomes necessary to install to a path other than the default).