faulthandler: use _PyTime_t rather than double for timeout (#4139)

Use the _PyTime_t type rather than double for the faulthandler
timeout in dump_traceback_later().

This change should fix the following Coverity warning:

CID 1420311:  Incorrect expression  (UNINTENDED_INTEGER_DIVISION)
Dividing integer expressions "9223372036854775807LL" and "1000LL",
and then converting the integer quotient to type "double". Any
remainder, or fractional part of the quotient, is ignored.

    if ((timeout * 1e6) >= (double) PY_TIMEOUT_MAX) {

The warning comes from (double)PY_TIMEOUT_MAX with:

    #define PY_TIMEOUT_MAX (PY_LLONG_MAX / 1000)