Raise NotImplementedError for to_datetime with z format

[mroeschke]

Description

Avoids timezone information from being dropped in to_datetime when the z directive is provided

In [1]: import cudf

In [2]: fmt = '%Y-%m-%d %H:%M:%S %Z'
   ...: dates = ['2010-01-01 12:00:00 UTC', '2010-01-01 12:00:00 UTC']

In [3]: cudf.to_datetime(dates, format=fmt)
Out[3]: DatetimeIndex(['2010-01-01 12:00:00', '2010-01-01 12:00:00'], dtype='datetime64[ns]')

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[wence-]

Suggested change

	["2020-01-01 00:00:00 UTC"], format=f"%Y-%m-%d %H-%M-%S %{code}"
	["2020-01-01 00:00:00 UTC"], format=f"%Y-%m-%d %H:%M:%S %{code}"

Format string doesn't match the provided date string without it (and once timezones are implemented, we would expect this test to raise ValueError because of it).

[mroeschke]

Thanks for the catch

[wence-]

Note: This is fine, but I'll just note that %%Z is a valid string for strptime that means "look for a literal %Z". So this replacement (and the existing %f replacement) is not robust against some of the wackier formats a user might provide.

[wence-]

Thanks @mroeschke

[mroeschke]

/merge