Here's a possible fix for this issue;  it parses a number by first stripping leading 
whitespace and any sign, then passing the remaining string to the system strtod.

This patch also fixes another bug in pystrtod.c, where ValueError is not raised on a 
hex float with a leading + sign:

>>> float('0x3')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for float(): 0x3
>>> float('+0x3')  # expect ValueError
3.0