python: Stop using basestring to test for strings

The base class basestring doesn't exist in Python 3. Use string_types
from six instead.

Change-Id: I7e84903fb7dd4a0af7ae4e9f4ec2e54338f212bb
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15998
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Juha Jäykkä <juha.jaykka@arm.com>

python: Replace deprecated repr syntax

Change-Id: I5f9538cf2ca5ee17c51e7c5388d3aef363fcfa54
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15989
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

python: Switch to using open instead of file

Python 3 doesn't support the file(name, mode) syntax which has been
deprecated in favour of open.

Change-Id: I35ef8690d97a5243860a64ff985fd22fa86253f1
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15985
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

python: Make exception handling Python 3 safe

Change-Id: I9c2cdfad20deb1ddfa224320cf93f2105d126652
Reviewed-on: https://gem5-review.googlesource.com/c/15980
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

grammar: better encapsulation of a grammar and parsing
This makes it possible to use the grammar multiple times and use the multiple
instances concurrently. This makes implementing an include statement as part
of a grammar possible.

ply grammar: Fixup Tokenizer class so you can get lexer arguments

ply: add a base class called Grammar that encapsulates a ply grammar