devel/py-convertdate: Fix broken dependencies

convertdate does not [1] depend on pyephem, which was replaced with
pymeeus in 2.2.0.

Dependency astro/py-pymeeus landed in ports r520157 as a new port.

This issue was identified while QA'ing pyephem dependents for bug 240735

[1] HISTORY.rst:* Replace pyephem, which is now in maintenance mode, with pymeeus.

PR: 240735
Approved by: portmgr (blanket(s): dependencies, runtime bugfix)
MFH: 2019Q4 (MFH with r520157)