fix download of mednist

[mibaumgartner]

Short Description

Please give a short summary of the main points of this PR

PR Checklist

PR Implementer

This is a small checklist for the implementation details of this PR.
If you submit a PR, please look at these points (don't worry about the RisingTeam
and Reviewer workflows, the only purpose of those is to have a compact view of
the steps)

If there are any questions regarding code style or other conventions check out our
summary.

• Implementation
• Docstrings & Typing
• Check __all__ sections and __init__
• Unittests (look at the line coverage of your tests, the goal is 100%!)
• Update notebooks & documentation if necessary
• Pass all tests
• Add the checksum of the last implementation commit to the Changelog

RisingTeam

RisingTeam workflow

• Add pull request to project (optionally delete corresponding project note)
• Assign correct label (if you don't have permission to do this, someone will do it for you.
  Please make sure to communicate the current status of the pr.)
• Does this PR close an Issue? (add closes #IssueNumber at the bottom if
  not already in description)

Reviewer

Reviewer workflow

• Do all tests pass? (Unittests, NotebookTests, Documentation)
• Does the implementation follow rising design conventions?
• Are the tests useful? (!!!) Are additional tests needed?
  Can you think of critical points which should be covered in an additional test?
• Optional: Check coverage locally / Check tests locally if GPU is necessary to execute

[review-notebook-app]

Check out this pull request on 

Review Jupyter notebook visual diffs & provide feedback on notebooks.

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[codecov]

Codecov Report

Merging #101 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #101   +/-   ##
=======================================
  Coverage   97.83%   97.83%           
=======================================
  Files          36       36           
  Lines        1384     1384           
=======================================
  Hits         1354     1354           
  Misses         30       30           

Flag	Coverage Δ
#unittests	97.83% <ø> (ø)

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