Digitalmarketplace-apiclient

Update to use python 3.9 and drop support for python 3.8.

Updated other packages so minimum Flask requirement is now v2

Update to use python 3.8 as python 3.6 is reaching end of life.

Dropping support for:
- python 3.6
- python 3.7

Mark this package as PEP 561 compatible. If you have dmapiclient as a dependency and run type checking, the type checker
will now check that you're using dmapiclient correctly. This might break your application's type checking if you're
using dmapiclient incorrectly.

All "attributes" of apiclient instances have been made private and replaced with read-only properties of the same name.
This is to encourage use of apiclients as objects that are unmutated after construction, making their use (hopefully)
threadsafe.

The attributes in question comprise `RETRIES`, `RETRIES_BACKOFF_FACTOR`, `RETRIES_FORCE_STATUS_CODES`, `base_url`,
`auth_token`, `enabled`, `timeout`. It seems unlikely that any code was mutating these values
post-construction/`init_app`, but it's worth doing a check before upgrading.

PR: [205](https://github.com/alphagov/digitalmarketplace-apiclient/pull/205)

The with_declarations parameter of find_framework_suppliers method of DataAPIClient now defaults to True instead of None. This will increase significantly the size of all replies using it.

PR: [152](https://github.com/alphagov/digitalmarketplace-apiclient/pull/152)

We're now using urllib3's Retry util to retry failed requests made by the base api client. This has allowed us to
remove backoff from `make_iter_method` and so also remove the package from the api client's dependencies. It also means
that we no longer need `HTTPTemporaryError` so it's been removed.

Anything that uses the api client and has it's own backoff setup (such as some of our scripts)
will need to adjust (or remove) their config to prevent potentially making far too many retry requests.

Anything that uses `HTTPTemporaryError` will need to remove it. The only place I've found it is in a few one off scripts
in the scripts repo for backoff setup.