Add autopkgtest-pkg-pybuild.1 manual page symlink

Colin Watsonrequested to merge
cjwatson/dh-python:autopkgtest-pkg-pybuild-symlink into master

The program is pybuild-autopkgtest, but it's helpful to document it as autopkgtest-pkg-pybuild as well since that's what goes in the Testsuite field.

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