[BB-3057] Use BytesIO as unicodecsv expects a bytestream, not unicode

[lgp171188]

This fixes the error thrown when trying to download the list of all
users, staff and instructors from the Sysadmin dashboard

Reference: unicodecsv README file

Testing instructions:

• In the Juniper devstack, enable the Sysadmin dashboard by setting the ENABLE_SYSADMIN_DASHBOARD feature flag to true in /edx/etc/lms.yml and restart the LMS.
• Log in to the LMS as a user with superuser permissions.
• Navigate to the Sysadmin dashboard by clicking on the link in the top nav bar.
• In the Users section, try to download a list of all the users in CSV file format. An exception complaining TypeError: string argument expected, got 'bytes' will be thrown.
• In the Staffing and enrollment section, try downloading the staff and instructor list in a CSV file format. A similar exception will be thrown.
• In a sandbox/production Juniper environment, a 500 internal server error response page will be shown and the same exception will be logged in the LMS logs.
• Checkout the source branch of this PR.
• Repeat the previous two steps and verify that the files can be downloaded and no errors are thrown.
• Also verify that the downloaded files contain the expected data.

[farhaanbukhsh]

👍

Have tested this and this works fine, I could reproduce the issues and verify the fix.

• I tested this: Verify the branch on devstack and saw it was giving the issue, used this branch and I could see it fixing it. Tried downloading file and verified the data
• I read through the code
• I checked for accessibility issues
• Includes documentation Doesn't require the documentation
• I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository.