Remove deprecations for v0.3

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6cbfe5c) 87.70% compared to head (0e6a287) 89.21%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #181 +/- ## ========================================== + Coverage 87.70% 89.21% +1.51% ========================================== Files 15 15 Lines 3244 3134 -110 Branches 748 723 -25 ========================================== - Hits 2845 2796 -49 + Misses 288 234 -54 + Partials 111 104 -7 ``` | [Files](https://app.codecov.io/gh/PyPSA/linopy/pull/181?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA) | Coverage Δ | | |---|---|---| | [linopy/constraints.py](https://app.codecov.io/gh/PyPSA/linopy/pull/181?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L2NvbnN0cmFpbnRzLnB5) | `91.17% <ø> (+2.72%)` | :arrow_up: | | [linopy/expressions.py](https://app.codecov.io/gh/PyPSA/linopy/pull/181?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L2V4cHJlc3Npb25zLnB5) | `89.67% <ø> (+0.07%)` | :arrow_up: | | [linopy/io.py](https://app.codecov.io/gh/PyPSA/linopy/pull/181?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L2lvLnB5) | `87.90% <100.00%> (+8.97%)` | :arrow_up: | | [linopy/variables.py](https://app.codecov.io/gh/PyPSA/linopy/pull/181?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=PyPSA#diff-bGlub3B5L3ZhcmlhYmxlcy5weQ==) | `92.36% <ø> (+1.47%)` | :arrow_up: |

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