Trac #17131: Improve accuracy of polytopes.regular_polygon()

The function `polytopes.regular_polygon()` uses a hard-coded
approximation of `pi` (3.14159265359) which has an error of 466 ulp.
Moreover, it uses Python `float`s instead of `RR`.

Note: there are actually a lot of these fishy approximations (like
`QQ(1618033)/1000000` for the golden ratio). Since this ticket is
intended to avoid a doctest failure in #17130, I only fixed
`regular_polygon()`.

URL: http://trac.sagemath.org/17131
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun