bpo-37412: os.getcwdb() now uses UTF-8 on Windows (GH-14396)

bpo-37412: os.getcwdb() now uses UTF-8 on Windows (GH-14396)

The os.getcwdb() function now uses the UTF-8 encoding on Windows,
rather than the ANSI code page: see PEP 529 for the rationale. The
function is no longer deprecated on Windows.

os.getcwd() and os.getcwdb() now detect integer overflow on memory
allocations. On Unix, these functions properly report MemoryError on
memory allocation failure.