Comment 2 for bug 1652113

Hi,
and thank you for your report.
The version is from zesty, I just installed the same package and it triggered as is.

From your log:
Setting up maas-rack-controller (2.1.3+bzr5573-0ubuntu1) ...
Traceback (most recent call last):
  File "/usr/sbin/maas-rack", line 85, in <module>
    main()
  File "/usr/sbin/maas-rack", line 81, in main
    run()
  File "/usr/sbin/maas-rack", line 52, in run
    from provisioningserver.__main__ import main
  File "/usr/lib/python3/dist-packages/provisioningserver/__main__.py", line 8, in <module>
    import provisioningserver.boot.install_grub
  File "/usr/lib/python3/dist-packages/provisioningserver/boot/__init__.py", line 387, in <module>
    PXEBootMethod(),
  File "/usr/lib/python3/dist-packages/provisioningserver/boot/__init__.py", line 275, in __init__
    assert issubclass(type(self.template_subdir), Optional[str])
  File "/usr/lib/python3.5/typing.py", line 770, in __subclasscheck__
    raise TypeError("Unions cannot be used with issubclass().")
TypeError: Unions cannot be used with issubclass().

@Maas Team - please take a look this actually should never have passed installability tests, but then it could have been some sort of side effect by another update.
To trigger you can just init a new lxd container and run "apt-get install maas-rack-controller"