Rewrite dict literal for files in tests/groupby & ./pandas/tests/dtypes 
- [ ] xref #38138
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
identify unnecessary dict call - rewrite as a literal
Rewrites as dictionary literals for the following files:

 ./pandas/tests/dtypes/test_inference.py
./pandas/tests/groupby/test_function.py
./pandas/tests/groupby/transform/test_transform.py

