I've added the quick opt out from https://fedoraproject.org/wiki/Changes/Avoid_usr_bin_python_in_RPM_Build#Quick_Opt-Out
Note that even the python3 tests for argcomplete use /usr/bin/python, i.e. Python 2. When the python2-argcomplete subpackage and the Python2 Build dependency are removed, tests fail. This means the software is not tested to run on systems without Python 2 (such as adefault install of Fedora).
This bug appears to have been reported against 'rawhide' during the Fedora 29 development cycle. Changing version to '29'.
Fixed in https://src.fedoraproject.org/rpms/python-argcomplete/c/af6af413a023c443a2c595ef655f540c195c1b77?branch=master by Igor.