fix: Replace positional Series indexing with .iloc in PSARIndicator

[dwaddle]

Fixes a FutureWarning (and eventual crash in future pandas versions) in PSARIndicator._run() in ta/trend.py.

Line 1030 used `self._psar[i] = high2` (deprecated positional integer indexing on a pandas Series). All 12 surrounding assignments in the same method already correctly use `.iloc[i]`. This single inconsistent line caused the warning reported in multiple issues.

## Fixes
Closes #354
Closes #357
Closes #348

## Change
```python
# Before (deprecated)
self._psar[i] = high2

# After
self._psar.iloc[i] = high2

Testing

Manually verified PSARIndicator runs without FutureWarning on a 60-bar OHLC dataset containing a trend reversal (which triggers the affected code path).

[dwaddle]

"Note: the ci/circleci: test_py36 failure is pre-existing and also fails on the current master
│ branch (verified via the GitHub status API). Our change is limited to a single line —
replacing
│ self._psar[i] with self._psar.iloc[i] — which is identical in behavior but correct for pandas
│ >= 2.0."