python-aodntools: Incorrect use of datetime.utcnow()

[mhidas]

Acceptance criteria

1. All uses of datetime.utcnow() have been replaced with datetime.now(timezone.utc)

Notes

• datetime.utcnow() is currently used to set the creation date (global attribute and in file names) in all timeseries_products code, and in ncwriter/imos_template.py
• This function can potentially return the system's local time as if it was UTC (see warning in datetime docs, or in more detail in this blog). This will lead to at most an 11-hour offset, which is not a huge problem, but it would be easy to fix.
• From datetime docs:

the recommended way to create an object representing the current time in UTC is by calling datetime.now(timezone.utc)

[mhidas]

This has been fixed for timeseries_products.
For ncwriter there was also an update required in the unittests, and an additional issue with Python 3.5 - so I've left this for later.