Use absolute imports

[nolze]

Related to #61

[codecov-commenter]

Codecov Report

Merging #63 (270a447) into master (fdd2a0c) will not change coverage.
The diff coverage is 96.42%.

@@           Coverage Diff           @@
##           master      #63   +/-   ##
=======================================
  Coverage   89.52%   89.52%           
=======================================
  Files          14       14           
  Lines        1241     1241           
=======================================
  Hits         1111     1111           
  Misses        130      130           

Impacted Files	Coverage Δ
msoffcrypto/__init__.py	88.46% <83.33%> (ø)
msoffcrypto/__main__.py	45.28% <100.00%> (ø)
msoffcrypto/format/doc97.py	88.55% <100.00%> (ø)
msoffcrypto/format/ooxml.py	80.70% <100.00%> (ø)
msoffcrypto/format/ppt97.py	98.90% <100.00%> (ø)
msoffcrypto/format/xls97.py	88.48% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fdd2a0c...270a447. Read the comment docs.