1.5.4

This is a patch release with only a few minor changes, including:

• Made harness.charm raise before begin (@PietroPasotti in #838)
• Use Storage.index instead of deprecated Storage.id (@benhoyt in #854)
• Add JujuVersion.has_secrets() (@beliaev-maksim in #855)
• Reinstate reversion of add_relation_unit ValueError to warning (@benhoyt in #856)

See the full list of commits here.

The next release will likely be 2.0.0 with Juju secrets support, and switching from minimum Python version 3.5 to 3.8.