Simplify packaging configuration

[jkassemi]

We should be able to use setuptools' default package discovery mechanism without modification, since it supports both LICENSE and README.rst inclusion without the need for a separate MANIFEST.in.

This addresses a bug I encountered caused by a separate tests package being installed by pip as part of flanker's installation.

Here's a fresh python install, with just pip in the site-packages:

james@Jamess-MBP-2 tmp-flanker % pyenv virtualenv tmp-flanker-fix
Looking in links: /var/folders/4h/2nsnlv397t92cv1fg6h1y2k40000gn/T/tmpppand0ob
Requirement already satisfied: pip in /Users/james/.pyenv/versions/tmp-flanker-fix/lib/python3.12/site-packages (24.0)
james@Jamess-MBP-2 tmp-flanker % pyenv local tmp-flanker-fix
(tmp-flanker-fix) james@Jamess-MBP-2 tmp-flanker % python -c "import site; print(site.getsitepackages())"
['/Users/james/.pyenv/versions/tmp-flanker-fix/lib/python3.12/site-packages']
(tmp-flanker-fix) james@Jamess-MBP-2 tmp-flanker % ls /Users/james/.pyenv/versions/tmp-flanker-fix/lib/python3.12/site-packages
pip			pip-24.0.dist-info

After installing flanker, notice the tests directory in site-packages:

(tmp-flanker-fix) james@Jamess-MBP-2 tmp-flanker % pip install flanker
...
Successfully installed WebOb-1.8.7 attrs-23.2.0 cffi-1.16.0 chardet-5.2.0 cryptography-42.0.5 flanker-0.9.11 idna-3.7 ply-3.11 pycparser-2.22 regex-2024.4.28 six-1.16.0 tld-0.13
(tmp-flanker-fix) james@Jamess-MBP-2 tmp-flanker % ls /Users/james/.pyenv/versions/tmp-flanker-fix/lib/python3.12/site-packages
WebOb-1.8.7.dist-info			attrs-23.2.0.dist-info			cryptography				idna-3.7.dist-info			pycparser				six.py
__pycache__				cffi					cryptography-42.0.5.dist-info		pip					pycparser-2.22.dist-info		**tests**
_cffi_backend.cpython-312-darwin.so	cffi-1.16.0.dist-info			flanker					pip-24.0.dist-info			regex					tld
attr					chardet					flanker-0.9.11.dist-info		ply					regex-2024.4.28.dist-info		tld-0.13.dist-info
attrs					chardet-5.2.0.dist-info			idna					ply-3.11.dist-info			six-1.16.0.dist-info			webob

And confirming the test directory is indeed coming from flanker:

(tmp-flanker-fix) james@Jamess-MBP-2 tmp-flanker % pip show -f flanker
Name: flanker
Version: 0.9.11
Summary: Mailgun Parsing Tools
Home-page: https://www.mailgun.com/
Author: Mailgun Technologies Inc.
Author-email: admin@mailgunhq.com
License: Apache 2
Location: /Users/james/.pyenv/versions/tmp-flanker-fix/lib/python3.12/site-packages
Requires: attrs, chardet, cryptography, idna, ply, regex, six, tld, WebOb
Required-by:
Files:
  flanker-0.9.11.dist-info/INSTALLER
  flanker-0.9.11.dist-info/LICENSE
  flanker-0.9.11.dist-info/METADATA
  flanker-0.9.11.dist-info/RECORD
  flanker-0.9.11.dist-info/REQUESTED
  flanker-0.9.11.dist-info/WHEEL
  flanker-0.9.11.dist-info/top_level.txt
  flanker-0.9.11.dist-info/zip-safe
  flanker/__init__.py
  ...
  flanker/mime/message/threading.py
  flanker/mime/message/utils.py
  flanker/utils.py
  tests/addresslib/__init__.py
  tests/addresslib/__pycache__/__init__.cpython-312.pyc
  tests/addresslib/__pycache__/address_test.cpython-312.pyc
  ...
  tests/mime/message/part_test.py
  tests/mime/message/scanner_test.py
  tests/mime/message/threading_test.py
  tests/mime/message/tokenizer_test.py
(tmp-flanker-fix) james@Jamess-MBP-2 tmp-flanker %