Configure step of package installation throws a python syntax error

[Colin Brough]

Clean install of 19.04. Adding packages, including Google commandline API tools, which pulls in 'python-protorpc-standalone'. Install fails with the following error:

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mica.cmb > sudo apt install python-protorpc-standalone
[sudo] password for cmb:
Reading package lists... Done
Building dependency tree
Reading state information... Done
python-protorpc-standalone is already the newest version (0.9.1-3).
0 to upgrade, 0 to newly install, 0 to remove and 0 not to upgrade.
2 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n]
Setting up python3-protorpc-standalone (0.9.1-3) ...
  File "/usr/lib/python3/dist-packages/protorpc/remote.py", line 579
    return async_method(self.async, *args, **kwargs).response
                                 ^
SyntaxError: invalid syntax

  File "/usr/lib/python3/dist-packages/protorpc/remote_test.py", line 634
    self.assertEquals(rpc, stub.async.remote_method(request))
                                    ^
SyntaxError: invalid syntax

dpkg: error processing package python3-protorpc-standalone (--configure):
 installed python3-protorpc-standalone package post-installation script subprocess returned error exit status 1
dpkg: dependency problems prevent configuration of python3-googlecloudapis:
 python3-googlecloudapis depends on python3-protorpc-standalone; however:
  Package python3-protorpc-standalone is not configured yet.

dpkg: error processing package python3-googlecloudapis (--configure):
 dependency problems - leaving unconfigured
No apport report written because the error message indicates it's a follow-up error from a previous failure.
                            Errors were encountered while processing:
 python3-protorpc-standalone
 python3-googlecloudapis
E: Sub-process /usr/bin/dpkg returned an error code (1)
mica:cmb.~ >

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