Require Python 3.8+ and drop the BPC conversion for 1.5.0 #2051

The owner ruled on #2040: 1.5.0 makes a full turn to Python 3.8+, and BPC is no longer maintained. Today requires-python = ">=3.6, <4", and BPC is what makes that possible. A 3.7 sdist build exits 0 but leaves modules that don't compile (#2040 has the measurements).

Work:

• pyproject.toml: set requires-python = ">=3.8". Drop the bpc-f2format, bpc-poseur and bpc-walrus requirements and the long BPC note above them. Drop markers that are always true from 3.8 on, such as scapy's python_version >= '3.7'. Bring the classifiers in line.
• setup.py: remove refactor() and the build hooks that call it, keeping whatever else those hooks do.
• pcapkit/utilities/compat.py: remove the gates that can no longer be taken, such as < (3, 6).
• vermin.ini: change targets to 3.8. Whether vermin becomes blocking stays the separate question noted in lint.yml.
• Docs: every passage stating 3.6 or 3.7 support or describing the BPC conversion.

Needs you: the 3.7 KTLO legs. The KTLO ruling keeps EOL versions as non-blocking legs. With >=3.8, though, pip refuses to install on 3.7 at all, so those legs could only prove that refusal.

• (a) Recommended: drop "3.7" from .github/python-versions.toml, together with its EXPECTED entries in coverage-comment.yml (ci: post Unit Tests push runs to Discussions (#2030) #2036). The KTLO legs for 3.8–3.10 stay.
• (b) Keep the 3.7 legs as a by-design refusal, and update their EXPECTED entries to pip's new message.