pkg,util: allow to keep running on python 3.9 by using the external importlib_metadata package
tsio/staircase: get rid of the historycache

We trade off a little time gain for a huge memory pressure decrease.
The .history calls are too dangerous.
migrate: update the version at a more opportune moment
Added tag 0.19.3 for changeset ee1580cfea71
http/client: handle the http 413 code
schema: simplify the group creation
http/client: handle 403 as unauthorized
http/client/update_metadata: return None on success
http/client: raise on server query crash
util/ts: turn this into a full-fledged subtype of str

This will make the life of the Python API users easier.
tsio/block_staircase: do not use the .history call

It will blow up the machine memory. Doing .get calls is better.
migrate/fix-groups-metadata: be more tolerant
tsio/find: provide the series kind

We need to discombobulate the .find method to allow
it to be extended.
api/find: fix the type annotation
api/find: typo fix in the docstring
Added tag 0.19.2 for changeset 478df8a23b02
build: fix pip invocation
pkg: force a better version-parser version