gh-139871: Optimize bytearray construction with encoding (#142243)

When a `str` is encoded in `bytearray.__init__` the encoder tends to
create a new unique bytes object. Rather than allocate new memory and
copy the bytes use the already created bytes object as bytearray
backing. The bigger the `str` the bigger the saving.

Mean +- std dev: [main_encoding] 497 us +- 9 us -> [encoding] 14.2 us +- 0.3 us: 34.97x faster

```python
import pyperf

runner = pyperf.Runner()

runner.timeit(
    name="encode",
    setup="a = 'a' * 1_000_000",
    stmt="bytearray(a, encoding='utf8')")
```