webapp: Pin django and typing-extensions to buster-backports versions
webapp: Pin django and typing-extensions to buster-backports versions

Related to T2303

SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site,SoftwareHeritage/puppet-swh-site