Use 'B' instead of 'b' to parse/build char value.

[masamitsu-murase]

Issue:

• pickle.load raises OverflowError if loaded content includes PyFrameObject, whose f_executing is set to SLP_FRAME_EXECUTING_INVALID, i.e. -1.

Solution:

• Use 'B' instead of 'b' to parse/build char value when frame object is pickled.

How to reproduce this issue:

• The following code causes OverflowError unexpectedly.

  import pickle
  import stackless
  import sys
  
  try:
     raise RuntimeError()
  except:
     traceback_object = sys.exc_info()[2]
  a = pickle.dumps(traceback_object)
  b = pickle.loads(a)
  c = pickle.dumps(b)
  d = pickle.loads(c)
  # => This raises "OverflowError: unsigned byte integer is less than minimum".

Details:

• f_executing in PyFrameObject is declared as char, which might be signed depending on implementation.
  Therefore, 'B' should be specified when Py_BuildValue and PyArg_ParseTuple is called.
• If 'b' is specified instead of 'B' and f_executing is set to SLP_FRAME_EXECUTING_INVALID, PyArg_ParseTuple in frame_setstate in Stackless\pickling\prickelpit.c raises OverflowError.
  This is raised in convertsimple in Python\getargs.c because SLP_FRAME_EXECUTING_INVALID is less than 0.