release(1.2.8): update mf6 version (modflowpy#132)

* Add latest version of mfusg_gsi (v2.1.1).
* Update test_request.py::test_latest_assets with latest asset list
* Update pymake calls in GH Actions workflows to use working-directory

release(1.2.7) update MODFLOW 6 version to 6.4.1 (modflowpy#128)

Update download._request_get() and dowload._request_header() to improve
ability to retry url requests. Improved flakyness of autotest/test_requests.py.
Update to usgsprograms.export_json() to handle directory paths appended to
fpth by separating it into a appdir and file_name. Also writing sorted code.json
and code.md.

release(1.2.6): update mf6 version (modflowpy#123)

Rename autotests with more descriptive names

fix(setup): add usgsprograms.txt to setup.py installation (modflowpy#103

)