[mlir][python] Add __{bool,float,int,str}__ to bindings of attributes.

This allows to use Python's `bool(.)`, `float(.)`, `int(.)`, and
`str(.)` to convert pybound attributes to the corresponding native
Python types. In particular, pybind11 uses these functions to
automatically cast objects to the corresponding primitive types wherever
they are required by pybound functions, e.g., arguments are converted to
Python's `int` if the C++ signature requires a C++ `int`. With this
patch, pybound attributes can by used wherever the corresponding native
types are expected. New tests show-case this behavior in the
constructors of `Dense*ArrayAttr`.

Note that this changes the output of Python's `str` on `StringAttr` from
`"hello"` to `hello`. Arguably, this is still in line with `str`s goal
of producing a readable interpretation of the value, even if it is now
not unambiously a string anymore (`print(ir.Attribute.parse('"42"'))`
now outputs `42`). However, this is consistent with instances of
Python's `str` (`print("42")` outputs `42`), and `repr` still provides
an unambigous representation if one is required.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D158974