🐛 sparse.linalg: fix several LinearOperator issues

[jorenham]

This fixes several issues in scipy.sparse.linalg.LinearOperator (docs), and closes #703.

• worked around a mypy issue (overloaded __new__ ignored when introducing __init__ python/mypy#17251) that caused mypy to falsely reject calls to LinearOperator, and will now correctly return a _CustomLinearOperator.
• .adjoint() and .H now return the correct LinearOperator specialization
• fixed contravariance issue in the LinearOperator.__new__ annotations for the custom mat* functions.
• avoid returning dtype union types for complex input in the public LinearOperator method.