ci: add a check for tabs

ci: add a check for tabs

Surprisingly, none of the standard tools complain about tabs in the
code, at least in some locations. I expected that python3 would always
complain, or at least tabnanny, or pyflakes… Let's roll a simple custom
check.

Use '-P' to request perl-compatible regular expressions even the user
has a different default configures so that '\t' works as expected.