Fix for stricter encoding names in latest MicroPython: UTF-8 → utf-8

[oliver-joos]

Since some commits were merged on 2026-07-20 MicroPython treats encoding names more strict:
Now only "utf-8", "utf8" or "ascii" are valid - lower-case!

micropython/micropython@11d002be
micropython/micropython@a62626a5
micropython/micropython@ba9ea94 ← docs and tests

This bug will cause a LookupError at runtime with latest MicroPython:

>>> "test".encode("UTF-8")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
LookupError: unknown encoding: UTF-8
>>> "test".encode("utf-8")
b'test'

This fix won't break compatibility to older MicroPython versions.