[mlir][python] Mark operator== overloads as const

This resolves some warnings when building with C++20, e.g.:
```
llvm-project/mlir/lib/Bindings/Python/IRAffine.cpp:545:60: warning: ISO C++20 considers use of overloaded operator '==' (with operand types 'mlir::python::PyAffineExpr' and 'mlir::python::PyAffineExpr') to be ambiguous despite there being a unique best viable function [-Wambiguous-reversed-operator]
                        PyAffineExpr &other) { return self == other; })
                                                      ~~~~ ^  ~~~~~
llvm-project/mlir/lib/Bindings/Python/IRAffine.cpp:350:20: note: ambiguity is between a regular call to this operator and a call with the argument order reversed
bool PyAffineExpr::operator==(const PyAffineExpr &other) {
                   ^
```

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D147018