Switch setuptools_scm to write version.py file

[djhoese]

See #1509 for details. This should remove any overhead of using pkg_resources when importing satpy.

• Closes Replace pkg_resources usage with version.py file #1509

[ghost]

DeepCode's analysis on #217318 found:

• ℹ️ 1 minor issue. 👇

Top issues

Description	Example fixes
Unused version imported from satpy.version as __version__ Occurrences: __init__.py:21 __init__.py:26 __init__.py:27	🔧 Example fixes

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[codecov]

Codecov Report

Merging #1512 (2173180) into master (2df64af) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1512   +/-   ##
=======================================
  Coverage   91.62%   91.63%           
=======================================
  Files         246      246           
  Lines       36008    36004    -4     
=======================================
- Hits        32993    32991    -2     
+ Misses       3015     3013    -2     

Flag	Coverage Δ
behaviourtests	4.38% <100.00%> (-0.01%)	⬇️
unittests	92.17% <ø> (ø)

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Impacted Files	Coverage Δ
satpy/__init__.py	100.00% <100.00%> (+11.76%)	⬆️

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[mraspaud]

Ok so what happens to the version number when:

• one installs from github
• one has made changes to the code and commited locally?
• one creates a sdist or bdist from a local branch with new commits?

[djhoese]

This PR doesn't change anything about the existing behavior as far as local installations except for performance. The version number still doesn't update unless you re-install. This should fix the github zip/tarball installation now since that was previously not fully enabled (missing dependency). That means someone installing from a github release I think.

[mraspaud]

ok

[mraspaud]

LGTM, thanks for looking into this! sounds good with some import optimisation.