[RavSinghChandan]

SVG 2 and CSS allow opacities as percentages, but fill-opacity="50%" raised ValueError: could not convert string to float: '50%'. Also, fill-opacity and stroke-opacity are clamped to 0-1 as the spec says, but opacity used a bare float(), so opacity="1.5" raised ValueError: 1.5 not in range [0.0, 1.0] later on.

clamp_float() (only used for opacities) now reads a trailing % as a fraction, and opacity goes through the same clamp_float(0.0, 1.0) as the other two. Added cases to test_svg_attribute_conversion.

Checklist:

• A unit test is covering the code added / modified by this PR
• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder (N/A)
• A mention of the change is present in CHANGELOG.md
• This PR is ready to be merged

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