strip: a route to allow stripping series from the http client
make the api generator open to extension

Instead of the plain `blueprint` function returning a flask blueprint,
we provide an `httpapi` object with attributes:

* bp (the blueprint)
* api
* ns

the two later being necessary to add new routes.
Added tag 0.9.1 for changeset da343e12abc9
test: adjust following change in tshistory

We also constrain the minimum tshistory version.
pkg: prepare 0.9.1
test/log: do a get with `insertion_date`
update: with a null diff, tshistory now returns an empty series
tests: repair supervision handling
Added tag 0.9.0 for changeset c8a5109d043e
test/formula-components: adjust for last minute tshistory-formula api change
util: move `handler` there

To make it accessible to e.g. tshistory_client (tests).
blueprint/supervision: fail on formula
blueprint: supervision/edited also supports the tshpack format
blueprint: supervision/edited support

json only, tsh-pack in the next commit.
test: handle the supervision plugin
conftest: simpler handler implementation
test: use util.has_formula
formula: provide the formula_components api end point
util/onerror: log rather thann print

prints in threads tend to be not well captured ...