Fix datetime imports

[pnuu]

In many places, we have used from datetime import datetime. This is bad, since it masks the module for further imports:

from datetime import datetime
from datetime.timezone import utc

Sorting these the other way around will cause complaints from flake8/ruff.

This PR changes all of these import to import datetime as dt, so we need on the single import to access other items:

import datetime as dt

dt.datetime(1980, 3, 19)
dt.timedelta(minutes=42)
now = dt.datetime.now(dt.timezone.utc)

This is a change that makes implementing the timezone aware datetimes of https://github.com/pytroll/satpy/issues/2752 easier.

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] Fully documented
• [ ] Add your name to AUTHORS.md if not there already

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.48611% with 26 lines in your changes missing coverage. Please review.

Project coverage is 95.94%. Comparing base (aeb3391) to head (3b5e62c). Report is 507 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/writers/awips_tiled.py	33.33%	4 Missing :warning:
satpy/readers/aapp_l1b.py	60.00%	2 Missing :warning:
satpy/readers/amsr2_l2_gaasp.py	33.33%	2 Missing :warning:
satpy/readers/ghrsst_l3c_sst.py	0.00%	2 Missing :warning:
satpy/readers/scatsat1_l2b.py	33.33%	2 Missing :warning:
satpy/readers/seviri_l1b_icare.py	60.00%	2 Missing :warning:
satpy/readers/vii_base_nc.py	60.00%	2 Missing :warning:
satpy/tests/features/steps/steps-load.py	0.00%	2 Missing :warning:
satpy/readers/caliop_l2_cloud.py	50.00%	1 Missing :warning:
satpy/readers/hrit_jma.py	80.00%	1 Missing :warning:
... and 6 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2787      +/-   ##
==========================================
+ Coverage   94.57%   95.94%   +1.37%     
==========================================
  Files         379      379              
  Lines       53886    53842      -44     
==========================================
+ Hits        50964    51661     +697     
+ Misses       2922     2181     -741     

Flag	Coverage Δ
behaviourtests	4.09% <1.38%> (+<0.01%)	:arrow_up:
unittests	96.04% <95.81%> (+0.87%)	:arrow_up:

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[coveralls]

Pull Request Test Coverage Report for Build 8783919300

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 8781051256:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls