Lessons learned switching from Python 3.6 to 3.7

If you use synth from ports-mgmt/synth and switch from Python 3.6 to 3.7 after having updated the ports tree and after synth have updated its own flavour index, you might see strange error messages such as:

net/samba410 scan aborted because dependency could not be located.
py37-dnspython>=1.15.0:dns/py-dnspython@py37 (required dependency of net/samba410) does not exist.
Scan of local/host1-localbase failed, it will not be considered.

dns/bind-tools scan aborted because dependency could not be located.
py37-ply>=0:devel/py-ply@py37 (required dependency of dns/bind-tools) does not exist.
Scan of local/host2-localbase failed, it will not be considered.

dns/bind-tools scan aborted because dependency could not be located.
py37-ply>=0:devel/py-ply@py37 (required dependency of dns/bind-tools) does not exist.
Scan of local/host3-localbase failed, it will not be considered.

devel/llvm80 scan aborted because dependency could not be located.
sphinx-build-3.7:textproc/py-sphinx@py37 (required dependency of devel/llvm80) does not exist.
Scan of local/host4-localbase failed, it will not be considered.

Encountered issue with local/host5-localbase or its dependencies
  => py37-certbot>0:security/py-certbot@py37 (required dependency of local/host5-localbase) does not exist.
Scan of local/host5-localbase failed, it will not be considered.

Encountered issue with local/host6-localbase or its dependencies
  => py37-pip>0:devel/py-pip@py37 (required dependency of local/host6-localbase) does not exist.
Scan of local/host6-localbase failed, it will not be considered.

Simply answer yes when synth asks you if it should recreate its flavour index.

If synth doesn’t ask you this question, simply touch(1) /usr/ports/Makefile or some other Makefile in the ports tree, and rerun synth and let it recreate its flavour index.