Check the obj type before calling the in operation

[tongbaojia]

• Type check fix
• The object can be of type byte instead of PDFStream.
  Without this type check, a python TypeError can be raised.
  In the case that raised the error, the obj turns out to be of class bytes.
• Fixes Type check for obj in PDFPageInterpreter #441

How Has This Been Tested?

Test passing locally using this branch.

Checklist

• I have added tests that prove my fix is effective or that my feature
  works
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or I am verified that this
  is not necessary
• I have updated the readthedocs documentation or I
  verified that this is not necessary
• I have added a consice human-readable description of the change to
  CHANGELOG.md

[pietermarsman]

I don't like that we scatter all these isinstance(obj, PDFStream) checks over the code base. But up until now that seems to be the fix for these kind of issues, so for now its ok.

Thanks for figuring this out.