json is simplejson since Python 2.6. Piglit requires Python >= 3.6.

From https://pypi.org/project/simplejson:
"simplejson is the externally maintained development version of the json
library included with Python (since 2.6)."

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/piglit/-/merge_requests/733>