PdfFileReader load cleanup

[MatthiasValvekens]

Description of the changes

Clean up some of the init logic in PdfFileReader (without fundamentally changing the fact that the xref table is processed during __init__), and in particular, for encrypted files, we now defer loading of the security handler to a later point. As a side effect, this means that problems with the security handler will be raised during object access and/or calls to .encrypt()/.decrypt() rather than during __init__, which is hopefully a bit more user-friendly. It also exposes the encryption dictionary as a property on the PdfFileReader class.
Closes #332.

Caveats

Technically, the removal of the .read() method on PdfFileReader is a breaking change. Practically, it's not, since calling that method out of order would already have broken all sorts of things anyway.

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ddf26f8) 98.81% compared to head (9578947) 98.82%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #343   +/-   ##
=======================================
  Coverage   98.81%   98.82%           
=======================================
  Files         113      113           
  Lines       16259    16270   +11     
=======================================
+ Hits        16067    16079   +12     
+ Misses        192      191    -1     

Flag	Coverage Δ
unittests	98.82% <100.00%> (+<0.01%)	⬆️

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[benjamin-awd]

LGTM -- I can confirm that encrypt_dict is now accessible as a class property 🎉

If you want, you could probably dump _get_encryption_params() and expose encrypt_dict directly using the property decorator like so: https://github.com/MatthiasValvekens/pyHanko/pull/346/files#diff-b31a4a5713e1dec7776e9bdc1372e73085050e9d9f7061a26d37586eb403afc5R207-R208 (seems to test fine locally, but unsure if this will have other unintended effects)

[MatthiasValvekens]

Yeah, fair enough. Yesterday me thought we might as well preserve the _get_encryption_params method while adding the property, but on reflection I agree it's kind of pointless.