Fix ABI mid-latitude cyclone demo downloading wrong number of files

[djhoese]

This fixes a couple issues pointed out about the demo functions. The first is that my glob pattern was a little too relaxed and was picking up newly generated file versions. I'm not really sure how I didn't run into this myself. Second was a workaround for a python 2.7 incompatibility. We don't support python 2 anymore so it could be removed. Lastly, flake8 now complains about function calls inside function definitions so my channels=range(1, 17) was flagged. That was fixed too.

I didn't add any tests for the bug because I'm not really sure how to do that. It all depends on what is actually available on the server and I'd like to avoid contacting GCP every time we run tests.

• Closes get_us_midlatitude_cyclone_abi in satpy.demo fails  #1291
• Closes update _makedirs in satpy.demo #1289
• Tests added
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.002%) to 90.081% when pulling a8a08f9 on djhoese:bugfix-demo-abi-midlatitude into f80c568 on pytroll:master.

[codecov]

Codecov Report

Merging #1295 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1295   +/-   ##
=======================================
  Coverage   90.07%   90.08%           
=======================================
  Files         218      218           
  Lines       31580    31576    -4     
=======================================
- Hits        28447    28444    -3     
+ Misses       3133     3132    -1     

Impacted Files	Coverage Δ
satpy/demo/__init__.py	97.29% <100.00%> (+2.17%)	⬆️

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[mraspaud]

LGTM