Fix extra border around range slider bar

[brisvag]

• fix part 2 of QSS theming inconsistencies with sliders #338
• fix Styling QRangeSlider #292

Extracted from #345 since part of that PR is not working, but this works on its own.

This only fixes the extra "border" around the range bar in the middle of the slider.

Before:

After:

from superqt import QRangeSlider, QLabeledDoubleRangeSlider, QDoubleRangeSlider, QLabeledRangeSlider
from qtpy.QtWidgets import QWidget, QVBoxLayout, QSlider
from qtpy.QtCore import Qt

w = QWidget()
l = QVBoxLayout()
w.setLayout(l)
QSS = """
QSlider::groove:horizontal {
    background-color: cyan;
    height: 10px;
}
QSlider::handle:horizontal {
    background-color: magenta;
    width: 10px;
}
QRangeSlider {
    qproperty-barColor: green;
}
"""

for cls in (QRangeSlider, QLabeledRangeSlider, QDoubleRangeSlider, QLabeledDoubleRangeSlider):
    slider = cls(Qt.Orientation.Horizontal)
    slider.setStyleSheet(QSS)
    value = (20, 80)
    slider.setValue(value)
    l.addWidget(slider)
w.show()

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.81%. Comparing base (595b284) to head (1ad5878).

Files with missing lines	Patch %	Lines
src/superqt/sliders/_range_style.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #347      +/-   ##
==========================================
- Coverage   84.85%   84.81%   -0.05%     
==========================================
  Files          49       49              
  Lines        3909     3911       +2     
==========================================
  Hits         3317     3317              
- Misses        592      594       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.