textproc/py-regex: Fix LICENSE

• While I'm here, add LICENSE_FILE

from pyproject.toml:
license = "Apache-2.0 AND CNRI-Python"
license-files = ["LICENSE.txt"]

Approved by: portmgr (blanket)
Reference: https://github.com/mrabarnett/mrab-regex/blob/hg/pyproject.toml#L13-L14