remove requirements.txt and references to it

[veenstrajelmer]

• [x] remove requirements.txt and requirements_dev.txt, they are superfluous since we moved from setup.py to pyproject.toml in https://github.com/Deltares/ddlpy/pull/61
• [x] remove reference in MANIFEST.in
• [x] replace references in .travis.yml (1x) and tox.ini (3x) with toml-to-req like in dfm_tools: https://github.com/Deltares/dfm_tools/blob/a4b2af357bf056cf56228049511307d5c38925f3/.github/workflows/pytest-py39-mindeps.yml#L25-L26
• [x] tox.ini and .travis.yml can also be removed
• [x] test build process