Use a single word space between sentences. In the nineteenth century,
which was a dark and inflationary age in typography and type design,
many compositors were encouraged to stuff extra space between sentences.
Generations of twentieth century typists were then taught to do the
same, by hitting the spacebar twice after every period. Your typing
as well asyour typesetting will benefit from unlearning this quaint
Victorian habit. As a general rule, no more than a single space is
required after a period, colon, or any other mark of punctuation.
Larger spaces (e.g., en spaces) are *themselves* punctuation.
(Some people go as far as to recommend less than word space after a period or comma, since the blank area above the punctuation mark contributes to the total visual white space.)
Anyway, in the grand scheme of things this is really insignificant :) The fact that PEP8 exists at all is just one of the things making Python great!