fix: expose ChannelFile encoding

[vaibhavjnf]

Summary

• expose the UTF-8 encoding on execnet channel file wrappers
• define the attribute once on the shared ChannelFile base
• cover both public Channel.makefile() modes with a regression test

This prevents text-stream consumers from raising AttributeError when an
execnet channel file replaces sys.stdout.

Verification

• real popen gateway reproduction with remote sys.stdout replacement
• uv run pytest -q testing/test_channel.py
• 279 passed, 217 skipped

Fixes #161

Implementation and review were assisted by OpenAI Codex. I reproduced the
failure and verified the final behavior and test suite locally.

[RonnyPfannschmidt]

The next major version of execnet will no longer be monkeypatching stdio