Rcpchgrowth

*Breaking change*
Removes support for python 3.8
Updates dependencies
Updates setup.py
Removes measurement advice string for <23 weeks gestation
Deprecated `package_utils` and replace with `importlib`
Add support for local development in docker using `s/up` (as per RCPCH development playbook)

Improvements include:
1. refactor centile value returned to 1 decimal place, rather than integer. There were inconsistencies with floats returning if centile < 1 or >99, but integers otherwise
2. bug fix in the fictional child function, which returned all dates as python datetimes, rather than dates. This was leading to validation errors in the response object in the server

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Updates Measurement object to return sds and centile values in plottable_data element

Rename start server files

Fix for centile line overlaps 2y and 42 weeks