Marrow.package

This release includes minor bumps, corrections, and improvements to package automation, migrates to GitHub Actions for CI/CD, future-proofs by protecting against underscore-prefixed attribute access, reduces test fragility, and implements package-relative file path lookup.

Minor version bump to Marrow Package to accommodate an upstream bump to `typeguard`.

* Python 3 modernization.
* Removed Python 2 support code, notably, exhaustive canonical name search.
* Eliminated future imports and compat module usage, unnecessary encoding declarations, etc.
* Python 3 native namespace use. Note: this is directly incompatible with any Marrow package that is compatible with Python 2.
* Type Annotations and in-development (run without optimizations) type validation / typechecking.
* Minimum Python version supported has been updated to Python 3.5.
* Added `pre-commit` configuration and removed unused `pytest` extensions.
* Eliminated Tox usage, expanded Travis to include Python 3.7, and removed excessive Travis scripts.
* Reduced the fragility of certain tests to the presence of globally installed packages on test worker nodes.
* Most commonly utilized utilities have been hoisted from their deep import paths to the top level, e.g. `name`, `load`, and `traverse` may now be imported from `marrow.package` directly.
* Optimizations including the elimination of unnecessarily repeated iteration.

* **Deprecated Python 2.6 and 3.3.** While no particular backwards incompatible change was made; as setuptools no longer supports these versions, these versions are now hard/impossible to test.

* **Allow extensions to declare exclusions.** Flags that must not be defined for the extension to be usable.