Added ability to test if xls97 is encrypted

[NPDW]

Added ability to test if xls97 is encrypted. Refactored the is_encrypted to be more class-based. Also slightly refactored Xls97File to be more similar to Doc97File

[codecov-io]

Codecov Report

Merging #11 into master will decrease coverage by 0.37%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
- Coverage   34.37%   33.99%   -0.38%     
==========================================
  Files          12       12              
  Lines         736      753      +17     
==========================================
+ Hits          253      256       +3     
- Misses        483      497      +14

Impacted Files	Coverage Δ
msoffcrypto/__main__.py	0% <0%> (ø)	⬆️
msoffcrypto/format/doc97.py	31.96% <50%> (+0.16%)	⬆️
msoffcrypto/format/ooxml.py	50.53% <50%> (-0.02%)	⬇️
msoffcrypto/format/xls97.py	15.06% <6.25%> (-0.97%)	⬇️

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[nolze]

Thanks!

I added minor changes after your commits:

• As _BIFFStream class I imlemented is quite an ad-hoc utility at present, I'd prefer to keep it for internal use only. We'll be able to make a better one in future.
• Seek Xls97File's data.workbook stream to zero before each use.
• Bump version to 4.6.0 by introducing is_encrypted() method.

I'll merge this PR in a few days. Please let me know if you have any thoughts.