Bugfix/fs encode implementation

[techalchemy]

Fix fsencode and fsdecode backports

• Mirrors the new python 3.7 implementation
• Taken from vistir (my other library) -> discussion over at
  Add support for proper filesystem encodings sarugaku/vistir#54
• Fixes Failures on Windows #13
• Fixes Backport changes from Python 3.6 #6 (I think?)
• Adds surrogatepass high bytes for testing the surrogatepass error handler which is implemented natively on windows as of python 3.6+
• Differs from the native implementations on python >3.0,<3.6 on windows in that it relies on the surrogatepass handler and opts to write unicode/utf-8 rather than the native handlers

[codecov]

Codecov Report

Merging #16 into master will decrease coverage by 8.05%.
The diff coverage is 69.51%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   96.55%   88.49%   -8.06%     
==========================================
  Files           5        5              
  Lines         174      226      +52     
  Branches       27       32       +5     
==========================================
+ Hits          168      200      +32     
- Misses          2       20      +18     
- Partials        4        6       +2

Impacted Files	Coverage Δ
tests/test_extra.py	85.18% <61.29%> (-14.82%)	⬇️
src/backports/os.py	86.66% <74.5%> (-7.01%)	⬇️
tests/test_helpers.py	100% <0%> (+4.16%)	⬆️

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