Support negative step in xfrange to match frange

[uttam12331]

Summary

xfrange is documented as "Same as frange, but generator-based" and shares frange's exact argument handling (including the range()-style start/stop swap). But its loop hard-codes the ascending condition:

cur = start
while cur < stop:
    yield cur
    cur += step

so a negative step yields nothing. frange supports a negative step — its docstring pins it:

>>> frange(5, 0, step=-1.25)
[5.0, 3.75, 2.5, 1.25]

whereas the equivalent xfrange call returned an empty generator:

>>> tuple(xfrange(5, 0, step=-1.25))
()          # before this fix; should equal frange's output

Fix

Choose the loop condition based on the sign of step:

-    while cur < stop:
+    while (cur < stop) if step > 0 else (cur > stop):

Tests

Added a negative-step doctest to xfrange mirroring frange's:

>>> tuple(xfrange(5, 0, step=-1.25))
(5.0, 3.75, 2.5, 1.25)

The existing positive-step doctest (xfrange(1, 3, step=0.75)) is unchanged and still passes. I verified xfrange now matches frange across all of frange's documented cases (ascending, empty, and the negative-step case).

[mahmoud]

Good catch, appreciate the effort. #443 reworked xfrange more broadly. Keep up the good work!