AttributeError when evaluating "%"

[gerritholl]

Describe the bug

When parsing the expression % using UnitRegistry.parse_expression, pint throws an AttributeError.

To reproduce

import pint
ureg = pint.UnitRegistry()
ureg.parse_expression("%")

Expected behaviour

I'm not sure what to expect. Maybe 0.01? ureg.parse_expression("1%") gives 1, but I'm not sure if that is accurate.

Actual results

Traceback (most recent call last):
  File "/home/gholl/checkouts/protocode/mwe/pint-ureg.py", line 3, in <module>
    ureg.parse_expression("%")
  File "/data/gholl/miniconda3/envs/py39/lib/python3.9/site-packages/pint/registry.py", line 1263, in parse_expression
    return build_eval_tree(gen).evaluate(
AttributeError: 'NoneType' object has no attribute 'evaluate'

Environment info

The latest pint master (Pint-0.18.dev3+g26e3a8d).

[hgrecco]

See #429 for the reason why this is not possible and reference therein for fixes.