Type check for obj in PDFPageInterpreter

[tongbaojia]

Bug report

• python type errors are raised when the object is not of type PDFStream.
  The obj type can be byte. There is no type check when using the functional call in.
  https://github.com/pdfminer/pdfminer.six/blob/develop/pdfminer/pdfinterp.py#L840

• The pdf file is attached as sample.pdf
python pdf2txt.py "sample.pdf" --pagenos "1"

• stacktrace.

Traceback (most recent call last):
  File "pdf2txt.py", line 188, in <module>
    sys.exit(main())
  File "pdf2txt.py", line 182, in main
    outfp = extract_text(**vars(A))
  File "pdf2txt.py", line 56, in extract_text
    pdfminer.high_level.extract_text_to_fp(fp, **locals())
  File "***/lib/python3.6/site-packages/pdfminer/high_level.py", line 86, in extract_text_to_fp
    interpreter.process_page(page)
  File "***/lib/python3.6/site-packages/pdfminer/pdfinterp.py", line 895, in process_page
    self.render_contents(page.resources, page.contents, ctm=ctm)
  File "***/lib/python3.6/site-packages/pdfminer/pdfinterp.py", line 908, in render_contents
    self.execute(list_value(streams))
  File "***/lib/python3.6/site-packages/pdfminer/pdfinterp.py", line 933, in execute
    func(*args)
  File "***/lib/python3.6/site-packages/pdfminer/pdfinterp.py", line 840, in do_EI
    if 'W' in obj and 'H' in obj:
TypeError: a bytes-like object is required, not 'str'

[tongbaojia]

sample.pdf

[pietermarsman]

I can replicate this issue using the newest pdfminer.six.