Fix TimeDeltaMissingUnitWarning in plot_airmass

[H-xie]

The function plot_airmass triggers the warning TimeDeltaMissingUnitWarning. This issue arises from unit checks implemented in newer versions of Astropy (refer to commit d8968b29a209fdaebd0f48ff30863718bca9db78).

The warning is generated because a time value without a unit is being added to an Astropy Time object. To resolve this warning, it is recommended to add the unit u.hour to the time value.

[m4cd4r4]

Clean fix for the TimeDeltaMissingUnitWarning. One thing worth checking:

In the datetime branch above this line, tzoffset is set to a bare float (from time.utcoffset().total_seconds() / 3600). If that value is later added to an astropy Time object, the same warning would appear for timezone-aware datetime inputs.

Worth adding * u.hour to the datetime branch too:

tzoffset = time.utcoffset().total_seconds() / 3600 * u.hour

Otherwise this is a straightforward one-liner that fixes a real warning. Looks like it needs a rebase to trigger CI.