Introduce `safer_eval`

[hristog]

tl;dr:

• Introduces a safer_eval function to replace eval (the former still relies on the latter, as ast.literal_eval is not suitable enough).
• Applies isort==4.3.21 to all files that have been changed.
• Removes unused line chunks = list().
• Updates deprecated import pandas.util.testing as tm to import pandas.testing as tm.