Still maintains an additional not logged in in a month check too.

Leaving 6 month values, at least for stats but may be reused in future.

The review discovered that the sync tool, when run on production, would never
succeed because the membership list of some groups/institutions was so large
that the API response exceeded the size limit imposed by API Gateway (due to
Lookup API returning detailed lists of members). This commit refactors the code
to fetch group/institution membership for each user and use that to populate
the membership set for each group/institution.

Use Python 3.9 in Dockerfile as new Python features are now used.

Update version of Flake8 so the checks are compatible with the newer version
of Python.

Remove redundant function `_extract_uid()`.

Modify Flake8 configuration to exclude the standard set of folders used across
projects.

Add settings for Lookup API and API Gateway credentials.

Add Lookup API operations to replicate the LDAP operations for gathering
information about users, groups and institutions.

Remove LDAP sync code and related configuration code.

Remove LDAP configuration options from the example file.

Remove LDAP library as a requirement it is no longer needed.

Remove LDAP references from documentation.

Add information to the README about how to provide API Gateway credentials.

Add YAML versions of API Gateway credentials file to `.gitignore`.