- singlespec automatic conversion of "Requires: python-*" does not
work for the packagename without "python-"
- only suggest external more intrusive plugins

- move to Python singlespec
- do not preload qtpy in runtests.py: avoids "Trace/breakpoint traps"
- skip newly failing tests because of meta change
- skip the slow mainwindow test again: the mainwindow fixture is
leaking file descriptors and causes undeterministic segfaults
gh#spyder-ide/spyder#13483