Fix: Include tox.ini in sdist for downstream testing

[ArnavBallinCode]

Issue Description

Fixes #43: The source distribution (sdist) package on PyPI is missing the tox.ini file, which affects downstream testing capabilities.

Environment Details

• httpbin version: 0.10.2
• Python version: 3.10
• OS: macOS 13.4
• Installation method: pip install httpbin

Expected Behavior

• tox.ini should be included in the source distribution
• File should be present when extracting the sdist package
• Running pip install -e .[test] should work without manual tox.ini creation

Actual Behavior

• tox.ini is missing from the sdist package
• Downstream users need to manually create tox.ini
• Makes testing more difficult for package maintainers

Changes Made

1. Added tox.ini to MANIFEST.in:

- include README.md LICENSE.ISC LICENSE.MIT AUTHORS test_httpbin.py
+ include README.md LICENSE.ISC LICENSE.MIT AUTHORS test_httpbin.py tox.ini

2. Updated tox.ini configuration:

[tox]
envlist = py37,py38,py39,py310,py311,py312,pypy3
isolated_build = True

[testenv]
deps =
    pytest>=6.0
    pytest-cov
    coverage
extras = test
commands =
    pytest {posargs:tests} --cov=httpbin
    coverage report

[pytest]
testpaths = tests
python_files = test_*.py
addopts = -v --tb=short

Verification Steps

1. Build the package locally:

python -m build --sdist

2. Verify tox.ini inclusion:

tar tzf dist/httpbin-0.10.2.tar.gz | grep tox.ini
# Output: httpbin-0.10.2/tox.ini

3. Test package installation:

pip install -e .[test]
tox -e py310  # Or your preferred Python version

Testing Evidence

$ tar tzf dist/httpbin-0.10.2.tar.gz | grep tox.ini
httpbin-0.10.2/tox.ini

$ python -m build --sdist
* Creating isolated environment...
* Building sdist...
Successfully built httpbin-0.10.2.tar.gz

Backwards Compatibility

• No breaking changes introduced
• Only adds missing file to distribution
• Maintains existing functionality
• Improves testing capabilities

Additional Notes

• Fixes downstream testing issues
• Makes package more maintainer-friendly
• Follows Python packaging best practices
• No changes to runtime behavior

Documentation Updates

No documentation updates required as this is a packaging fix.

Related Issues

• Closes sdist is missing tox.ini #43
• Improves package maintainability
• Helps downstream packagers