get_var_name does not work in python 3.14

When using python 3.14 `amaranth.tracer.get_var_name` fails to find the variable name. Everything seems to work with python 3.13. This can be seen in https://github.com/kuznia-rdzeni/transactron (just run `pytest` with python 3.14). Similar problem was found and fixed in litex: https://github.com/enjoy-digital/litex/pull/2407