prevent warnings when compiling with `gcc -Wextra`

prevent warnings when compiling with `gcc -Wextra`

The PyInt conversion functions generate two warnings when compiled under
`gcc -Wall -Wextra`:

1. comparison of unsigned expression < 0 is always false

2. signed and unsigned type in conditional expression

This patch fixes these problems by creating a new temporary variable
`is_unsigned`, which fixes problem 1, and by converting the ternary
return expression into a normal if/else branch, which fixes problem 2.