Fix _to_identifier

[matthiasdiener]

Fixes inducer/arraycontext#260.

I think running isidentifier on individual characters doesn't make too much sense; there are other occurrences of this in the code:

• grudge/grudge/dof_desc.py

  Lines 91 to 95 in 5b73a2b

  	def _to_identifier(name: str) -> str:
  	if not name.isidentifier():
  	return "".join(ch for ch in name if ch.isidentifier())
  	else:
  	return name

• https://github.com/inducer/arraycontext/blob/146c598c688b519eb4cc6be5966a56e2a6e6292e/arraycontext/impl/pytato/compile.py#L55-L56

[inducer]

https://pypi.org/project/pytools/2024.1.3/