Update Babel to >=2.9.1

[zenmonkeykstop]

A vulnerability in Babel 2.9.0 has been found that allows for arbitrary code execution, as described here: https://www.tenable.com/security/research/tra-2021-14

Initial analysis of the vulnerability indicates that it isn't exploitable in SecureDrop, but as a matter of course the dependency should be updated in the next release.