Convert `=&gt;` to the more semantic `&rarr;`.

`&rarr;` is defined in the HTML 4 spec [1]:

  <!ENTITY rarr CDATA "&#8594;" -- rightwards arrow, U+2192 ISOnum -->

Don't make these replacements in example Python code:

On Sun, Jan 06, 2013 at 05:47:00AM -0800, Greg Wilson wrote:
> On 2013-01-06 8:37 AM, W. Trevor King wrote:
> > On Sun, Jan 06, 2013 at 05:21:02AM -0800, Greg Wilson wrote:
> > > * Where '=>' appears as a string in a Python program to be printed,
> > > it should be left alone.
> >
> > Oops. Sloppy me not checking all my automatic replacements. How do
> > you feel about replacing the Python occurences with `\u2192'`?
>
> -1: I don't want us to have to explain Unicode escape sequences that
> early in the show.  If we could actually trust all the editors people
> use, and legacy tools like Cygwin's 'cat', to display non-7-bit
> characters correctly, I'd just put the ' →' directly in the source, but
> hey, I'd like a unicorn too...

[1]: http://www.w3.org/TR/html4/sgml/entities.html