Vendor compressed-rtf functionality to fix installation

[gordonhart]

A recent breaking change to setuptools breaks builds for projects relying on extract-msg via the transitive dependency compressed-rtf.

The compressed-rtf library appears to be unmaintained and has a small enough surface area to consider vendoring (porting its relevant functionality over to this repo).

Minimal repro:

$ virtualenv venv
$ source ./venv/bin/activate
$ pip install extract-msg
Collecting extract-msg
  Downloading extract_msg-0.54.0-py3-none-any.whl.metadata (15 kB)
Collecting olefile==0.47 (from extract-msg)
  Downloading olefile-0.47-py2.py3-none-any.whl.metadata (9.7 kB)
Collecting tzlocal<6,>=4.2 (from extract-msg)
  Downloading tzlocal-5.3.1-py3-none-any.whl.metadata (7.6 kB)
Collecting compressed-rtf<2,>=1.0.6 (from extract-msg)
  Downloading compressed_rtf-1.0.6.tar.gz (5.8 kB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... error
  error: subprocess-exited-with-error
  ... stack trace ...
          File "/private/var/folders/bf/l5_3dbwx6sg3jb_2x50znfx80000gp/T/pip-build-env-_jr9bsyz/overlay/lib/python3.13/site-packages/setuptools/dist.py", line 629, in _enforce_underscore
          raise InvalidConfigError(
          ...<3 lines>...
          )
      setuptools.errors.InvalidConfigError: Invalid dash-separated key 'description-file' in 'metadata' (setup.cfg), please use the underscore name 'description_file' instead.
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× Getting requirements to build wheel did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.

Related: delimitry/compressed_rtf#20

Edit: this was resolved in the v78.0.2 release of setuptools but the setuptools maintainers are planning to revisit and perhaps reintroduce this breakage in 2026.