gnulib-tool.py: Inline 'sed' invocations used on library files. * pygnulib/GLFileSystem.py (GLFileAssistant.__init__): Update type hints and docstrings to reflect changes necessary for using re.sub() instead of 'sed'. (GLFileAssistant.add_or_update): Use re.sub() instead of invoking 'sed'. * pygnulib/GLImport.py (GLImport.prepare): Update transformation variables to reflect changes to GLFileAssistant.