Remove Python 2 compatibility helpers: to_bytes, to_native, to_unicode

[lavr]

These three functions in emails/utils.py are Python 2 compatibility wrappers:

• to_native(x) — if bytes, decode to str; otherwise return as-is
• to_unicode(x) — same, plus str(x) for non-str/bytes
• to_bytes(x) — if str, encode to bytes; otherwise return as-is

On Python 3 they are trivial wrappers around .encode() / .decode(). They were already removed from message.py, signers.py, and utils.py itself in #194. Remaining call sites:

• emails/loader/ (helpers.py, local_store.py, __init__.py)
• emails/transformer.py
• emails/store/file.py (to_bytes only)

Each call can be replaced with direct .encode() / .decode() / isinstance check. Once all call sites are migrated, the functions and their overloads can be removed from utils.py.