bt2: field_class: rename range set Python class template to _range_set_pycls

bt2: field_class: rename range set Python class template to _range_set_pycls

For consistency, as this is the pattern used elsewhere.

Change-Id: Ifaaf82cab922da2c780d4af26580fa16974f7dcd
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2041
CI-Build: Francis Deslauriers <francis.deslauriers@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>