The shapely upper pin is causing issues when using PyPSA in an environment when a newer shapely version's feature is required.

Looking at the file blame, @FabianHofmann introduced the pin, possibly to handle deprecation warnings? Tests all pass and no shapely warning are raised in a version with this unpinned.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• Unit tests for new features were added (if applicable).
• A note for the release notes docs/release-notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.