pkgtools/pkglint: update to 20.3.1

Changes since 20.3.0:

URLs from cpan.metacpan.org should not be used as HOMEPAGE for Perl
packages since they don't provide an overview over the package, instead
the just list the distribution files.

Lines in doc/CHANGES may contain the pattern "Removed <pkgpath> version
<version>".  Before, only "Removed <pkgpath>" (without additional
information) or "Removed <pkgpath> successor <pkgpath>" were allowed.
Mentioning the last seen version number sounds useful.  It is not
cross-checked right now; that check may be added later.

(rillig)