Polyfile

This release switches to using `importlib` for loading resources. This fixes an issue if PolyFile is installed from an `sdist` into a discrete `venv` with resources stored separately from the code.

Documentation on extending PolyFile is also improved.

This release ends official support for Python 3.7, updates its use of [fickling](https://github.com/trailofbits/fickling) to a new API, and adds missing build dependencies to the source distribution.

Adds support for detecting Python pickle serializations.

Adds support for parsing HTTP/1.1 request headers.

Significant improvements to the libmagic DSL matching engine. PolyFile now supports the distinction between "text" and "binary" tests, similar to libmagic. This version of PolyFile has been validated against the ~1k file [Corkami test corpus](https://github.com/corkami/pocs).

Several new features were added to PolyFile's interactive debugger.

**Breaking Change**: Prior to this release, the default behavior of PolyFile was to output its matching results in JSON format (`--format json`). *As of version 0.5.0, PolyFile will now default to mimicking the output of libmagic's `file` command (`--format file`)*.

Updates to the command line interface to allow for greater flexibility in specifying output formats and output files. These updates are backwards-compatible, however, in the forthcoming v0.5.* release there will be a breaking change in the output: PolyFile will then default to the `mime` output format (to mimic the behavior of the `file` command) rather than the current `sbud` output.

This release also includes optimizations to the libmagic `search` and `string` tests which should result in significant speed improvements.