Pin cryptography to <2.5. Temporary workaround for #173.

[jaraco]

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)

#173

❓ What is the current behavior? (You can also link to an open issue here)

Tests are failing with Cryptography 2.5

❓ What is the new behavior (if this is a feature change)?

Install older versions until an underlying cause can be ascertained.

📋 Other information:

📋 Checklist:

• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences

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This change is 

[jaraco]

Superseded by #175.