> Why is that? python will work just fine if it can't write pyc files.

When I hacked around the _sysconfigdata.py issue, running the created python produced

Fatal Python error: Py_Initialize: Unable to get the locale encoding
Traceback (most recent call last):
  File "/mnt/hgfs/Repos/cpython-dirty/Lib/encodings/__init__.py", line 31, in <module>
  File "<frozen importlib._bootstrap>", line 1562, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1529, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 590, in _check_name_wrapper
  File "<frozen importlib._bootstrap>", line 1027, in load_module
  File "<frozen importlib._bootstrap>", line 1008, in load_module
  File "<frozen importlib._bootstrap>", line 566, in module_for_loader_wrapper
  File "<frozen importlib._bootstrap>", line 858, in _load_module
  File "<frozen importlib._bootstrap>", line 994, in get_code
  File "<frozen importlib._bootstrap>", line 1055, in _cache_bytecode
  File "<frozen importlib._bootstrap>", line 1078, in set_data
  File "<frozen importlib._bootstrap>", line 128, in _write_atomic
OSError: [Errno 5] Input/output error: '/mnt/hgfs/Repos/cpython-dirty/Lib/__pycache__/codecs.cpython-33.pyc.3076609376'
Aborted

I took this to be a failure to write the bytecompiled file.  (A regression?)  Running "./python -B" does work correctly for me.