bpo-41486: Fix initial buffer size can't > UINT32_MAX in zlib module (GH-25738)

* Fix initial buffer size can't > UINT32_MAX in zlib module

After commit f9bedb630e8a0b7d94e1c7e609b20dfaa2b22231, in 64-bit build,
if the initial buffer size > UINT32_MAX, ValueError will be raised.

These two functions are affected:
1. zlib.decompress(data, /, wbits=MAX_WBITS, bufsize=DEF_BUF_SIZE)
2. zlib.Decompress.flush([length])

This commit re-allows the size > UINT32_MAX.

* adds curly braces per PEP 7.

* Renames `Buffer_*` to `OutputBuffer_*` for clarity