Fixed BytesWarnings

[radarhere]

https://github.com/python-pillow/Pillow/actions/runs/3745012260/jobs/6358985335#step:8:4128

Tests/test_file_ppm.py::test_plain_invalid_data[P1\n128 128\n1009]
/opt/hostedtoolcache/Python/3.11.1/x64/lib/python3.11/site-packages/PIL/PpmImagePlugin.py:211: BytesWarning: str() on a bytes instance
raise ValueError(f"Invalid token for this mode: {bytes([token])}")

Tests/test_file_ppm.py::test_plain_ppm_token_too_long[P3\n128 128\n255\n012345678910]
/opt/hostedtoolcache/Python/3.11.1/x64/lib/python3.11/site-packages/PIL/PpmImagePlugin.py:245: BytesWarning: str() on a bytes instance
f"Token too long found in data: {half_token[:max_len + 1]}"

Tests/test_file_ppm.py::test_plain_ppm_token_too_long[P3\n128 128\n255\n012345678910 0]
/opt/hostedtoolcache/Python/3.11.1/x64/lib/python3.11/site-packages/PIL/PpmImagePlugin.py:251: BytesWarning: str() on a bytes instance
f"Token too long found in data: {token[:max_len + 1]}"

This PR fixes these BytesWarnings by changing the entire error messages to bytes.