Ltd-conveyor

- Fix parsing of the `GITHUB_HEAD_REF` environment variable in GitHub Actions.

- The `ltd upload` command now works in pull requests workflows on GitHub Actions.

- This release focuses on infrastructure improvements, and should not have a significant impact on users.

- This tool is officially tested and supported on Python 3.7 and 3.8. Python 3.6 is no longer officially supported.

- Removed pins on third-party dependencies, including boto3, requests, uritemplate, and click. In doing so, LTD Conveyor is easier to integrate into your existing Python environment.

- We have migrated from Travis CI to GitHub Actions for testing, linting, documentation builds and deployment, and deployments to PyPI. In addition to on-demand testing, GitHub Actions tests the package on a daily basis to ensure compatibility with third-party dependencies.

- Internally, we use tox for running tests, linters, and to build documentation. tox makes it possible for local test runs to use the same set up as CI.

- The codebase is now automatically formatted with black and isort. By using pre-commit, we ensure that formatting is always applied.

- The codebase now has type annotations, which are tested with mypy. Type annotations help ensure that APIs are explicitly defined and correctly used.

Bugs fixed

- The `--gh` option for `ltd upload` now supports tag and PR events, in addition to branches.

New features

- Added a `--gh` option to the `ltd upload` command to support usage in [GitHub Actions](https://help.github.com/en/actions).

New features

- LTD Conveyor can now upload new builds to S3 using Amazon's S3's presigned POST URL feature. This means that clients no longer need credentials for S3 — the LTD Keeper API server generates presigned POST URLs as part of the `POST /products/<product>/builds/` build registration step.
- The `ltd` command now uses the presigned POST-based URLs. The `--aws-id` and `--aws-secret` options have been removed, but the `--user` and `--password` options remain. If you use environment variables, there should be no change in how you use `ltd upload` (aside from not needing the `$LTD_AWS_ID` and `$LTD_AWS_SECRET` environment variables).

Bugs fixed

- In INFO-level and higher logging, the module path isn't displayed. Module paths are only shown in DEBUG-level logging where it's most useful.

Other changes

- Packaging is now done through a `setup.cfg` file and a `pyproject.toml` file (PEP 518).
- Updated Travis CI configuration to test with Python 3.8; also refactored the CI Pipeline with Travis CI stages.