fixed importing of 'utils' in 'slate.classes' so that it wouldn't clobber when users have another module called 'utils'

[violet4]

this is a better version than my previous #44, following that described by #32 (comment)

• I kept it simple for ease of review and merging
• my friend had a namespace clobber - slate's insert utils imported a different utils
• the import statement from . import utils isn't compatible in all situations - out of these two situations, it usually only works in one of them: running the program which contains from . import utils directly from the command line (e.g. python classes.py) vs importing the file which contains from . import utils as a module from other python code.

my original solution was import slate.utils as utils, but I think from slate import utils as described by @mvherweg in #32 (comment) is a better solution.

[violet4]

considering issue #51 and my PR is from nearly 6 years ago, i'll go ahead and close this PR.