macports package py36-matplotlib fails to install -- depends on obsolete macports package py36-pyobj-cocoa

[nealeyoung]

### Bug report

Trying to install py36-matplotlib from the current macports (macports version 2.5.4) fails, as py36-matplotlib depends on an obsolete package py36-pyobj-cocoa.

Here are macports instructions that demonstrate the problem

% sudo port selfupdate  # updates to MacPorts base version 2.5.4

% sudo port info py36-matplotlib
....
Library Dependencies: ... py36-pyobjc-cocoa ...
...

% sudo port info py36-pyobjc-cocoa
....
Description:          This port is obsolete.
...

% sudo port install py36-matplotlib +cairo 
--->  Computing dependencies for py36-matplotlib
The following dependencies will be installed:  py36-pyobjc-cocoa
Continue? [Y/n]: 
--->  Configuring py36-pyobjc-cocoa
Error: py36-pyobjc-cocoa is obsolete; please uninstall it.
Error: Failed to configure py36-pyobjc-cocoa: obsolete port
Error: See /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_python_py-pyobjc-cocoa/py36-pyobjc-cocoa/main.log for details.
Error: Follow https://guide.macports.org/#project.tickets to report a bug.
Error: Processing of port py36-matplotlib failed

Matplotlib version

• Operating system: Mac OSX 10.14.1
• Matplotlib version: py36-matplotlib @3.0.2
• Python version: Python 3.6.8 installed in /opt/local/bin/python

I believe all relevant packages were installed via macports.

The same bug seems to exist in other libraries such as py37-matplotlib.

[nealeyoung]

Sorry, please close this, it is a duplicate of ​https://trac.macports.org/ticket/57870