custom keyring data file path

[gzm55]

add an optional environment KEYRING_PROPERTY_SAGECIPHER_DATA to specify a custom keyring data file path.

close #7

[codecov]

Codecov Report

Merging #8 (7479bbe) into master (9fcc8a9) will increase coverage by 1.18%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   46.84%   48.02%   +1.18%     
==========================================
  Files           4        4              
  Lines         269      279      +10     
==========================================
+ Hits          126      134       +8     
- Misses        143      145       +2     

Impacted Files	Coverage Δ
sagecipher/keyring.py	87.09% <80.00%> (-3.38%)	⬇️

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