Fix Path slicing to match standard sequence semantics

[gaoflow]

Problem

Path.__getitem__ handles slicing by scaling the start/stop indices into
the path's internal (op, value) ops layout, but it never clamps out-of-range
bounds and mishandles negative steps. The result is a Path whose .values()
disagrees with the equivalent tuple slice, and empty slices produce a Path
with a misaligned ops tuple:

>>> from glom import Path
>>> p = Path('a', 'b', 'c')
>>> p.values()
('a', 'b', 'c')
>>> p[-5:].values()          # expected ('a', 'b', 'c')
('c',)
>>> p[::-1].values()         # expected ('c', 'b', 'a')
('a', 'b', 'c')
>>> Path('x', 'y')[-4:-3]    # expected Path() ; corrupt result

Integer indexing already clamps/raises correctly; only the slice branch is
affected. This is the sibling of the recent integer-index fix (GH-299).

Fix

Split the slice branch out and drive it with range(*i.indices(len(self))),
which yields exactly the component indices standard sequence slicing would
(clamped bounds, negative steps honored). The (op, value) pairs for those
components are then reassembled. The integer-index branch is unchanged.

Tests

test_path_slices_match_sequence_semantics checks clamped negative bounds,
negative steps, empty-slice validity, and an exhaustive cross-check of
Path(...)[key].values() == values_tuple[key] over paths of length 0–4 and a
grid of start/stop/step values.

pytest glom/test/test_path_and_t.py passes (the 3 unrelated test_cli.py
failures in my environment are a missing optional YAML dependency and reproduce
on a clean checkout).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (6fd4134) to head (b2d3775).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #301   +/-   ##
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  Coverage   98.22%   98.23%           
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  Files          27       27           
  Lines        4391     4407   +16     
  Branches      612      617    +5     
=======================================
+ Hits         4313     4329   +16     
  Misses         53       53           
  Partials       25       25           

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