Sorry, I'm -1 on this:  outside Fortran, using 'E' for the exponent marker seems to be near universal.  It just doesn't seem worth adding the extra complication to the Python code, or going through all the various places that expect an 'e' (float literals in Python source, inputs to the float function, Decimal and Fraction inputs, complex numbers, etc.) and trying to decide whether 'd' should be supported there too.

Even in Fortran, isn't it normal to output floats and doubles using the 'E' format specifier?