The encoding is not specified when opening the manifest file on Windows

[CKrawczyk]

When working with the aggregation code I found out that on Windows the default encoding used for opening a file is not UTF-8, and caused issues with non-English characters in the file. It could be the cause of the issues seen here: https://www.zooniverse.org/talk/18/673942

Could be fixed by adding encoding='utf-8' to open function call on this line:

panoptes-cli/panoptes_cli/commands/subject_set.py

Line 169 in 69b42da

with open(manifest_file, 'U') as manifest_f:

[adammcmaster]

The encoding option seems to have been added in Python 3, so I don't think we can do anything about this while we're will supporting Python 2.

When we do eventually fix this, we should probably add a command-line option to allow the user to specify the encoding rather than always forcing it to UTF-8 (but UTF-8 should probably be the default).