Core: Remove six.py and fix Python 3.13 compatibility

Delete mgear.core.six and mgear.rigbits.six (Python 2/3 compat
layer no longer needed). Replace all six imports: range is builtin,
string_types replaced with str, urllib uses stdlib directly, PY2
checks removed. Fix invalid escape sequences in core/string.py
docstrings that cause SyntaxWarning in Python 3.12+/Maya 2027.

Author: miquelcampos

release/scripts/mgear/anim_picker/gui.py

@@ -45,7 +45,6 @@
 
 from .handlers import __EDIT_MODE__
 from .handlers import __SELECTION__
-from mgear.core.six.moves import range
 
 # constants -------------------------------------------------------------------
 try:

release/scripts/mgear/anim_picker/widgets/picker_widgets.py

@@ -31,7 +31,6 @@
     python_handlers,
     maya_handlers,
 )
-from mgear.core.six.moves import range
 
 # constants -------------------------------------------------------------------
 SCRIPT_DOC_HEADER = """