Digitalmarketplace-content-loader

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

Dropping support for:
- python 3.6
- python 3.7

PR [149](https://github.com/alphagov/digitalmarketplace-content-loader/pull/149)
marks this package as PEP 561 compliant. If you have dmcontent as a dependency and run type checking, the type checker
will now check that you're using dmcontent correctly. This might break your application's type checking if you're using
dmcontent incorrectly.

PR [71](https://github.com/alphagov/digitalmarketplace-content-loader/pull/71)

Removal of support for `python3.3` and lower.

Upgrade `PyYAML` from 3.11 to 5.1.2 (see [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)).

`utils.TemplateField`'s `template` attribute is no longer a real jinja `Template`, and now only has a single method,
`render([context])`. On the plus side, `ContentLoader` trees should be deepcopy-able.

PR [59](https://github.com/alphagov/digitalmarketplace-content-loader/pull/59)

Removal of `python2` support.

Upgrade `flask` to from 0.10.1 to 1.0.2. This has breaking changes for flask apps and therefore has breaking changes for users relying on this package.
Apps should upgrade to `Flask==1.0.2` using the changelog here http://flask.pocoo.org/docs/1.0/changelog/#version-1-0-2 taking care to note
the breaking changes in [v1.0](http://flask.pocoo.org/docs/1.0/changelog/#version-1-0)