gis: use wkt for object identifyer (shapely >= v2.0 compat)

[codecov[bot]]

Codecov Report

Base: 72.47% // Head: 72.40% // Decreases project coverage by -0.07% :warning:

Coverage data is based on head (f31adf5) compared to base (ef0c863). Patch coverage: 77.77% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #270 +/- ## ========================================== - Coverage 72.47% 72.40% -0.08% ========================================== Files 19 19 Lines 1584 1587 +3 Branches 218 220 +2 ========================================== + Hits 1148 1149 +1 Misses 370 370 - Partials 66 68 +2 ``` | [Impacted Files](https://codecov.io/gh/PyPSA/atlite/pull/270?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA) | Coverage Δ | | |---|---|---| | [atlite/gis.py](https://codecov.io/gh/PyPSA/atlite/pull/270?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-YXRsaXRlL2dpcy5weQ==) | `89.84% <77.77%> (-0.68%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA)

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