Replace print() against self.stdout/stderr

[jrief]

Superseeds #539

replace print statements in management command against self.stdout, self.stderr as required by best Django practices.

[jrief]

@jaap3 could you please quickly review this pr for version 2.7.2.

[jaap3]

Thanks @jrief, the from django.utils.timezone import datetime, timedelta imports are still there, but I guess I cannot convince you on that. Everything else looks good to me, so to not obstruct a next release I'll just approve this as is.

[jrief]

@jaap3 just released 2.7.2.

Thanks for reminding me about releasing a new version.