Fix add_argparse_args raising TypeError with Python 3.6

[akihironitta]

What does this PR do?

Fixes #9535.

Does your PR introduce any breaking changes? If yes, please list them.

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[codecov]

Codecov Report

Merging #9554 (ac2196a) into master (86c0335) will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #9554   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         180     180           
  Lines       15090   15090           
======================================
  Hits        14019   14019           
  Misses       1071    1071           

[akihironitta]

The failing tests are irrelevant to the changes made here. Also, DeepSource is complaining that __init__ doesn't contain any code at:
https://github.com/PyTorchLightning/pytorch-lightning/blob/ac2196afd541ddd9faf234192083077cd50f816c/tests/utilities/test_argparse.py#L146

but a test class above also doesn't contain any code in __init__
https://github.com/PyTorchLightning/pytorch-lightning/blob/ac2196afd541ddd9faf234192083077cd50f816c/tests/utilities/test_argparse.py#L136

so I believe this PR is ready for your review now.