fix(multi heritage): Handle multi heritage using string join

This needs open-source/logilab-common!46
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5 jobs for !13 with topic/default/multi_heritage in 53 seconds (queued for 6 seconds)
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Status Name Job ID Coverage
  Lint
passed black #463445
lint black

00:00:23

passed check-manifest #463447
lint check-manifest

00:00:37

passed flake8 #463444
lint flake8

00:00:41

passed mypy #463446
lint mypy

00:00:48

 
  Tests
passed py3 #463448
tests py3

00:00:34