[bugfix] fix getversion_svn for Python 3.6+
- fix getversion_svn for Python 3.6+ which has a different value for struct_time.n_fields attribute; use len() instead.
- pathlib is mandatory for pywikibot. It is no longer necessary to have a fallback to os.
- remove getversion_svn deprecation warning
getversion_svn_setuptools uses svn_utils from setuptools or setuptools_svn. Both aren't included neither in Python 3 nor in Python 2. It is not usefull to recommend installing an external library to just get the version.
getversion_svn is still working and needed as a fallback if external packages aren't installed. Do no longer show a deprecation warning which is not appropriate.