Fix handling of paths with forward slashes on Windows

[djhoese]

I noticed with the UW SIFT application that paths returned by the file dialog (Qt5) always had forward slashes, even on Windows. This is expected behavior by Qt5. These forward slashes were causing Satpy to not recognize any of the filenames. While I could workaround this in SIFT, Satpy has other users in the past with this issue. I always thought this was a user problem, but now that I find out some applications/frameworks like Qt5 create paths like this, I think Satpy should have the fix.

The one thing I don't like about this PR is that it adds an extra step to get_filebase which is used multiple times in the reader loading process. I'm not sure if it is "better" to move this operation higher up in the reader loading functions for not. Additionally, I notice that match_filenames creates a list instead of returning a generator. It doesn't seem necessary to do this so I might change that too (feedback welcome).

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.03%) to 87.523% when pulling 7a0899e on djhoese:bugfix-windows-forward-slash into fa997a0 on pytroll:master.

[codecov]

Codecov Report

Merging #1049 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1049      +/-   ##
==========================================
+ Coverage   87.49%   87.52%   +0.02%     
==========================================
  Files         188      188              
  Lines       29238    29245       +7     
==========================================
+ Hits        25582    25596      +14     
+ Misses       3656     3649       -7

Impacted Files	Coverage Δ
satpy/tests/test_yaml_reader.py	99.57% <100%> (+0.43%)	⬆️
satpy/readers/yaml_reader.py	94.4% <100%> (ø)	⬆️
satpy/scene.py	89.91% <0%> (+0.17%)	⬆️
satpy/readers/__init__.py	94.56% <0%> (+1.2%)	⬆️

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