[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.

Bug: T253617
Bug: T132292
Change-Id: I66ae3f6a4ba5a7be343ec10c6f8e9671894763f5