fix: floor/ceil/round_ used as iteratees silently pass index as precision

[gaoflow]

Bug

pydash.floor, pydash.ceil, and pydash.round_ each accept an optional
precision argument. helpers.getargcount() counts all positional
parameters and therefore returns 2 for these functions. When callit()
invokes them inside iteriteratee() it passes (element, index, collection)
truncated to argcount=2, so the collection index is forwarded as precision.
This produces silently wrong results whenever these functions are used directly
as iteratees.

Reproducer

import pydash as _

# Reported expected: {6.0: 2, 4.0: 1}
_.count_by([6.1, 4.2, 6.3], _.floor)
# Actual:   {6.0: 1, 4.2: 1, 6.3: 1}
#
# floor(6.1, 0) == 6.0  ✓  (index 0 → precision 0, happens to be correct)
# floor(4.2, 1) == 4.2  ✗  (index 1 → precision 1, rounds to 1 decimal)
# floor(6.3, 2) == 6.3  ✗  (index 2 → precision 2, rounds to 2 decimals)

The same bug affects _.ceil and _.round_. Equivalent lambdas work fine
because lambda x: _.floor(x) only has one positional parameter.

Fix

Set _argcount = 1 on floor, ceil, and round_ after their definitions.
This is the existing optimization mechanism already used by iteratees returned
from utilities.iteratee() and the wrappers in functions.py; it tells
callit() to supply only the element value.

Normal two-argument use (floor(3.75, 1)) is completely unaffected.

Tests

Three new parametrize cases added to test_count_by — one per affected
function — covering the previously broken behavior.

[dgilland]

Thanks for the contribution, much appreciated! 👍