Problem trying to install Steam: ModuleNotFoundError: No module named 'apt_inst'

Question

When I try to install Steam, I get the following error:

$ sudo gdebi ~/steam.deb

Traceback (most recent call last):
  File "/usr/bin/gdebi", line 38, in <module>
    from GDebi.GDebiCli import GDebiCli
  File "/usr/share/gdebi/GDebi/GDebiCli.py", line 36, in <module>
    from .DebPackage import DebPackage, DscSrcPackage
  File "/usr/share/gdebi/GDebi/DebPackage.py", line 24, in <module>
    import apt.debfile
  File "/usr/lib/python3/dist-packages/apt/debfile.py", line 23, in <module>
    import apt_inst
ModuleNotFoundError: No module named 'apt_inst'

I already reinstalled python3, but the ModuleNotFoundError persists. How to fix it?