check_specifier() raises an exception when not passed as string

[plannigan]

dist.check_specifier() is used to validate that the given argument is valid. The documentation and the error message it produces clearly states the value should be a string. However, if the value is something other than a string, DistutilsSetupError is not raised. This is because SpecifierSet class .split() assuming that the value is a string. This causes:

AttributeError: 'lXXX' object has no attribute 'split'

In my case, it happened to be a list containing one valid specifier.

Instead of failing with this error, check_specifier() should raise DistutilsSetupError to clearly indicate that it was given a value of the wrong type.

This is related to #1869 in that it is hitting the same error. But, I'm suggesting the type be properly checked instead of allowing other types.