MAINT: Refactor eigh and eigvalsh and associated tests.

Do not convert the UPLO argument to a bytestring, it is not necessary.
Distribute parts of the eigh tests into the appropriate TestEigvalsh or
TestEigh test class.
Merge pull request #3982 from charris/refactor-eigh-eigvalsh

MAINT: Refactor eigh and eigvalsh and associated tests.