S3Boto3: Double localtime() call Exception on modified_time() when settings.USE_TZ=False

[piglei]

Current implemention of S3Boto3 backend respect USE_TZ settings on Django>=1.10. But when Django<1.10 users set USE_TZ to False, modified_time() will raise an ValueError:

ValueError: astimezone() cannot be applied to a naive datetime

That's because get_modified_time() already returns a naive datetime object, modified_time() call localtime on this naive object again.

[jschneier]

Fixed in #235.