ports-mgmt/portshaker: Add python as a build dependency

It is not really required, but without it, the run-time dependency on
mercurial is non-functional when the MERCURIAL option is on.

Does not impact the default packagage, and poudriere will catch the new
dependency if MERCURIAL=on so no need to bump PORTREVISION.

PR: 289133
Reported by: Tassilo Philipp <tphilipp@potion-studios.com>