gh-129813, PEP 782: Use PyBytesWriter in _io.FileIO.readall (#138901)

gh-129813, PEP 782: Use PyBytesWriter in _io.FileIO.readall (#138901)

Performance about the same, using the benchmark from gh-120754:

```
pyperf compare_to main.json pep782.json
read_file_small: Mean +- std dev: [main] 5.71 us +- 0.05 us -> [pep782] 5.68 us +- 0.05 us: 1.01x faster
read_file_large: Mean +- std dev: [main] 89.7 us +- 0.9 us -> [pep782] 86.9 us +- 0.8 us: 1.03x faster
read_all_rst_bytes: Mean +- std dev: [main] 926 us +- 8 us -> [pep782] 920 us +- 12 us: 1.01x faster
read_all_rst_text: Mean +- std dev: [main] 2.24 ms +- 0.02 ms -> [pep782] 2.17 ms +- 0.04 ms: 1.03x faster

Benchmark hidden because not significant (1): read_all_py_bytes

Geometric mean: 1.01x faster
```