pypsa does not work with Pyomo 6.4.3
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## Checklist

- [ ] I am using the current [`master`](https://github.com/pypsa/pypsa/tree/master) branch or the latest [release](https://github.com/pypsa/pypsa/releases). Please indicate: 0.21.1

## Describe the Bug

`from pyomo.core.expr.logical_expr import inequality` in `opt.py` produces an error with pyomo 6.4.3

`ImportError: cannot import name 'inequality' from 'pyomo.core.expr.logical_expr'` as it was moved to `pyomo.core.expr.relational_expr`

To fix this, the import should be `from pyomo.environ import inequality` as this is meant as the public import without assuming the internal package structure of pyomo which might change again in the future.
https://github.com/Pyomo/pyomo/blob/main/pyomo/core/tests/unit/test_relational_expr.py#L24
diff --git a/pypsa/opt.py b/pypsa/opt.py
index e709161d..0b2da39b 100644
--- a/pypsa/opt.py
+++ b/pypsa/opt.py
@@ -27,9 +27,8 @@ import tempfile
 from contextlib import contextmanager
 
 from pyomo.core.base.constraint import _GeneralConstraintData
-from pyomo.core.expr.logical_expr import inequality
 from pyomo.core.expr.numeric_expr import LinearExpression
-from pyomo.environ import Constraint, Objective, Var, minimize
+from pyomo.environ import Constraint, Objective, Var, inequality, minimize
 from six.moves import cPickle as pickle
 
 # =============================================================================
['pypsa/opt.py']