Conjure-python-client

* use latest conjure-python which requires only python 3.6

_No documented user facing changes_

| Type | Description | Link |
| ---- | ----------- | ---- |
| Fix | Fix type definitions of conjure types to always accept Type[DecodableType] | https://github.com/palantir/conjure-python-client/pull/114 |

| Type | Description | Link |
| ---- | ----------- | ---- |
| Improvement | Add py.typed marker | https://github.com/palantir/conjure-python-client/pull/109 |
| Break | Stop testing on python 2 | https://github.com/palantir/conjure-python-client/pull/110 |
| Break | Require python 3.8 | https://github.com/palantir/conjure-python-client/pull/110 |

_No documented user facing changes_

_No documented user facing changes_