Fix ResourceWarning 'unclosed file' if LockException is raised

[ahauan4]

Fixes ResourceWarning 'unclosed file' if LockException is raised

The following warning is raised:
ResourceWarning: unclosed file
Object allocated at (most recent call last):
File "portalocker\utils.py", lineno 174
return open(self.filename, self.mode, **self.file_open_kwargs)

[coveralls]

Coverage remained the same at 100.0% when pulling de9a7c4 on ahauan4:ahauan4-fix-ressource-warning-unclosed-file into a306f61 on WoLpH:develop.

[codecov]

Codecov Report

Merging #50 into develop will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           develop       #50   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          160       161    +1     
  Branches        13        13           
=========================================
+ Hits           160       161    +1     

Impacted Files	Coverage Δ
portalocker/utils.py	100.00% <100.00%> (ø)

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[wolph]

Good fix, thank you!