* [MRG] Add basic support for encrypted PDF files
Update API and CLI to accept ASCII passwords to decrypt PDFs
encrypted by algorithm code 1 or 2 (limited by support from PyPDF2).
Update documentation and unit tests accordingly.
Example document health_protected.pdf generated as follows:
qpdf --encrypt userpass ownerpass 128 -- health.pdf health_protected.pdf
Issue #162
* Support encrypted PDF files in python3
Issue #162
* Address review comments
Explicitly check passwords for None rather than falsey.
Correct read_pdf documentation for Owner/User password.
Issue #162
* Correct API documentation changes for consistency
Issue #162
* Move error tests from test_common to test_errors
Issue #162
* Add qpdf example
* Remove password is not None check
* Fix merge conflict
* Fix pages example
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