Figure.solar: Fix the bug that terminator datetime doesn't support fractional timezone offsets

[seisman]

Description of proposed changes

As reported in #4112 amd GenericMappingTools/gmt#9220, GMT's solar module doesn't accept fractional timezone offsets like 08:30. GMT always casts it to 8.

GMT already has a fix at GenericMappingTools/gmt#9221. This PR adds a workaround on the PyGMT side, by always converting timezone-aware datetime to UTC datetime before passing to GMT. With this workaround, PyGMT works well for any GMT versions.

This bug can be confirmed by the script below:

import pygmt
import datetime

fig = pygmt.Figure()
fig.basemap(region="d", projection="W0/15c", frame=True)
fig.solar(terminator_datetime="2020-01-01T01:02:03", pen="1p,black")
fig.solar(terminator_datetime="2020-01-01T01:02:03+00:30", pen="1p,yellow,-")
fig.solar(terminator_datetime="2020-01-01T01:02:03+01:00", pen="1p,red")
fig.solar(terminator_datetime="2020-01-01T01:02:03-01:00", pen="1p,blue")
fig.solar(
    terminator_datetime=datetime.datetime(
        2020, 1, 1, 1, 2, 3, tzinfo=datetime.timezone(datetime.timedelta(hours=2))
    ),
    pen="1p,lightred",
)
fig.solar(
    terminator_datetime=datetime.datetime(
        2020, 1, 1, 1, 2, 3, tzinfo=datetime.timezone(datetime.timedelta(hours=-2))
    ),
    pen="1p,lightblue",
)
fig.show()

Note the position of the yellow dashed line for a timezone offset +00:30.

Current main branch	This branch

Patches #4112.

[seisman]

With this workaround, we don't need the +z modifier (line 143), so this workaround can simplify the code and is not temporary. So I don't add a # TODO comment to remove it in future versions.