Fix PDF reading - part 1

[zhiltsov-max]

Motivation and context

Fixes #2139

How has this been tested?

Manual test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[nmanovic]

Please add a line into CHANGELOG as well.

  File "/home/django/cvat/apps/engine/views.py", line 46, in <module>
35129    from . import models, task
35130  File "/home/django/cvat/apps/engine/task.py", line 314
35131    elif isinstance(extractor, (MEDIA_TYPES['zip']['extractor'], MEDIA_TYPES['pdf']['extractor'])):
35132    ^
35133SyntaxError: invalid syntax

[zhiltsov-max]

WIP - Will fail after the first interpreter exit.