Add backup validation

[robin-nitrokey]

This patch adds the Backup and EncryptedBackup classes.
EncryptedBackup.parse can be used to parse and validate the format of a
backup file. EncryptedBackup.decrypt can be used to decrypt a backup
and validate the decrypted data.

This also adds validation of the backup file to the unit tests.

Fixes: #49

Depends on: #51

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (bd090a7) 79.36% compared to head (436ac8e) 79.42%.

❗ Current head 436ac8e differs from pull request most recent head 84970f5. Consider uploading reports for the commit 84970f5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
+ Coverage   79.36%   79.42%   +0.06%     
==========================================
  Files         782      783       +1     
  Lines       13387    13465      +78     
==========================================
+ Hits        10625    10695      +70     
- Misses       2762     2770       +8     

Files	Coverage Δ
nethsm/backup.py	89.47% <89.47%> (ø)

... and 1 file with indirect coverage changes

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[daringer]

first merge this and release then update the pynitrokey dependency to a release instead of a git url?