It looks like bug #676074[1]
I've found a suitable workaround of looking at the versions of python
I'm building layman for, and explicitly telling it to use a version of
Python other than 3.7 to run layman.
katipo2 ~ # EPYTHON=python3.7 layman -a pentoo
* Adding overlay...
* CLI: Errors occurred processing action add
* Exception caught installing repository "pentoo":
* unterminated character set at position 1
katipo2 ~ # EPYTHON=python3.6 layman -a pentoo
* Adding overlay...
* Running Git... # ( cd /var/lib/layman && /usr/bin/git clone
https://github.com/pentoo/pentoo-overlay.git /var/lib/layman/pentoo )
* Running Git... # ( cd /var/lib/layman/pentoo && /usr/bin/git config
user.name "layman" )
* Running Git... # ( cd /var/lib/layman/pentoo && /usr/bin/git config user.email "layman@localhost" )
* Successfully added overlay(s) pentoo.
1:
https://bugs.gentoo.org/676074