grass.script: Make grass_path keyword only in init (#3689)

This makes the grass_path parameter a keyword-only argument (PEP 3102). While this technically breaks the 8.0 API for 8.4, the documentation only shows grass_path as a keyword argument, grass_path is required to be a keyword argument when location and mapset are omitted, and given the automation triggered by grass_path=None, any usage of grass_path is likely limited (and the usage would have to be as positional argument to trigger the incompatibility).

python/grass/script/setup.py

@@ -268,7 +268,7 @@ def setup_runtime_env(gisbase=None, *, env=None):
     env["PYTHONPATH"] = path
 
 
-def init(path, location=None, mapset=None, grass_path=None):
+def init(path, location=None, mapset=None, *, grass_path=None):
     """Initialize system variables to run GRASS modules
 
     This function is for running GRASS GIS without starting it with the