Convert to PyQt6

[cjmayo]

Qt6Help.pc not installed until Ubuntu 22.10 (Kinetic).

[coveralls]

Pull Request Test Coverage Report for Build 4670936364

• 22 of 413 (5.33%) changed or added relevant lines in 24 files are covered.
• 10 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.009%) to 13.13%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
linkcheck_gui/help.py	1	2	50.0%
linkcheck_gui/main.py	0	2	0.0%
linkcheck_gui/recentdocs.py	2	4	50.0%
linkcheck_gui/updater.py	0	3	0.0%
linkcheck_gui/validator.py	0	3	0.0%
linkcheck_gui/editor_qsci.py	1	5	20.0%
linkcheck_gui/linkchecker_ui_debug.py	1	6	16.67%
linkcheck_gui/editor.py	1	8	12.5%
linkcheck_gui/editor_qt.py	0	7	0.0%
linkcheck_gui/syntax.py	0	8	0.0%
<!--	Total:	22	413	5.33%	-->

Files with Coverage Reduction	New Missed Lines	%
linkcheck_gui/lineedit.py	1	13.42%
linkcheck_gui/syntax.py	1	0%
linkcheck_gui/init.py	8	24.4%
<!--	Total:	10		-->

Totals
Change from base Build 4611043205:	0.009%
Covered Lines:	370
Relevant Lines:	2818

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 4681619155

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 22 of 413 (5.33%) changed or added relevant lines in 24 files are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.009%) to 13.13%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
linkcheck_gui/help.py	1	2	50.0%
linkcheck_gui/main.py	0	2	0.0%
linkcheck_gui/recentdocs.py	2	4	50.0%
linkcheck_gui/updater.py	0	3	0.0%
linkcheck_gui/validator.py	0	3	0.0%
linkcheck_gui/editor_qsci.py	1	5	20.0%
linkcheck_gui/linkchecker_ui_debug.py	1	6	16.67%
linkcheck_gui/editor.py	1	8	12.5%
linkcheck_gui/editor_qt.py	0	7	0.0%
linkcheck_gui/syntax.py	0	8	0.0%
<!--	Total:	22	413	5.33%	-->

Files with Coverage Reduction	New Missed Lines	%
linkcheck_gui/init.py	1	24.82%
linkcheck_gui/lineedit.py	1	13.42%
linkcheck_gui/syntax.py	1	0.0%
<!--	Total:	3		-->

Totals
Change from base Build 4611043205:	0.009%
Covered Lines:	370
Relevant Lines:	2818

---

💛 - Coveralls

[cjmayo]

Ubuntu 22.04 doesn't provide Qt6Help.pc, 22.10 does but without libexecdir; qhelpgenerator found in bin on 22.04, and libexec from 22.10.

Add a fallback if Qt6Help.pc doesn't contain libexecdir and now the workflows run on 22.04.