Setting tag sfa-3.1-21

Setting tag sfa-3.1-21
sfax509 command can run openssl x509 on all the parts of a gid
bugfix in sfi when running the discover subcommand
PEP8'ed a substantial part of the code
additional debug in chain verification
sfi myslice more robust - ignores broken slices
new sfi introspect
for myslice: the trust/ package should be python3-ready
* provided that m2crypto is
* although the devel version of m2crypto wroks just fine for us
* even if it's incomplete for other aspects that we do not care about