Bump pdf2image from 1.6.0 to 1.13.1 in /cvat/requirements (#1772)

Bumps [pdf2image](https://github.com/Belval/pdf2image) from 1.6.0 to 1.13.1.
- [Release notes](https://github.com/Belval/pdf2image/releases)
- [Commits](https://github.com/Belval/pdf2image/compare/v1.6.0...v1.13.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

cvat/requirements/base.txt

@@ -32,7 +32,7 @@ djangorestframework==3.11.0
 Pygments==2.6.1
 drf-yasg==1.17.0
 Shapely==1.7.0
-pdf2image==1.6.0
+pdf2image==1.13.1
 pascal_voc_writer==0.1.4
 django-rest-auth[with_social]==0.9.5
 cython==0.29.20