Enabling the tests and adding Build-Depends: python3-six makes the tests
fail as described in the upstream issue. There is a partly fix available.

Dear submitter,

as the package python-u2flib-server has just been removed from the Debian archive
unstable we hereby close the associated bug reports.  We are sorry
that we couldn't deal with your issue properly.

For details on the removal, please see https://bugs.debian.org/1126429

The version of this package that was in Debian prior to this removal
can still be found using https://snapshot.debian.org/.

Please note that the changes have been done on the master archive and
will not propagate to any mirrors until the next dinstall run at the
earliest.

This message was generated automatically; if you believe that there is
a problem with it please contact the archive administrators by mailing
ftpmaster@ftp-master.debian.org.

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Thorsten Alteholz (the ftpmaster behind the curtain)