chore: migrate to uv, src layout, and PEP 8 package name

[n3rada]

Hello 👋

I took some time to modernise the packaging of this project so it is easier to contribute to and install from.

What Changed

• Replaced setup.py, setup.cfg, requirements.txt, and requirements-dev.txt with a single pyproject.toml compliant with PEP 621, using uv_build as the build backend.
• Moved the package into a src/ layout, which is now the recommended structure for Python projects to avoid import shadowing issues.
• Renamed the package directory from SalesforcePy/ to salesforcepy/ to follow PEP 8 (lowercase package names).
• Updated all test files and docs to reflect the new import name (import salesforcepy as sfdc).
• Added pythonpath = ["src", "tests"] to the pytest config so tests resolve correctly without any install step.
• Added all co-authors from the module docstrings to pyproject.toml.
• Dropped from __future__ import absolute_import across all modules (Python 2 compatibility shim, not needed on Python 3).
• Dropped the try/except ImportError urllib compat block in sfdc.py for the same reason.
• Fixed two SyntaxWarning caused by invalid escape sequences (\**kwargs) in docstrings.
• Updated README.md with installation instructions for uv and pip.

All 79 existing tests pass without modification to the test logic itself.

Hope this is useful.

Happy to adjust anything.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @n3rada to sign the Salesforce Inc. Contributor License Agreement.

[n3rada]

This PR also resolves #74. By locking dependencies with uv, requests resolves to 2.34.2, well past the 2.32.0 security fix from that Dependabot bump.

[n3rada]

This PR also addresses #68. The pyproject.toml sets requires-python = ">=3.11,<4.0", formally dropping all EOL Python versions (3.5, 3.6, 3.7, 3.8, 3.9, 3.10) and setting the floor at the current stable release.

[n3rada]

Tangentially related to #67 as well: the old .travis.yml is now superseded. With pyproject.toml and uv in place, setting up a GitHub Actions workflow to replace it would be straightforward (uv run pytest is all it needs).