fix(python): Fix use of memoryview to write fill to the buffer builder

[paleolimbot]

At least one test is failing on pypy because of an unreleased buffer. This was due to the line:

memoryview(builder)[out_start : out_start + length] = b"\x01" * length

...which relied on the deletion of memoryview(builder) to release the builder buffer. One solution is:

with memoryview(builder) as mv:
  mv[out_start : out_start + length] = b"\x01" * length

However, we also have ArrowBufferAppendFill() in the C library, so instead, this PR wires that up and uses it (which should be faster, anyway).