Addresses SettingWithCopyWarning in read_csv_with_nan

[geoffreyangus]

This PR addresses the SettingWithCopyWarning that appears in integration tests using tests.integration_tests.utils.read_csv_with_nan.

SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  df[col].iloc[row] = np.nan

[github-actions]

Unit Test Results

       6 files  ±0         6 suites  ±0   2h 18m 6s ⏱️ + 7m 9s
2 798 tests ±0  2 763 ✔️ ±0    35 💤 ±0  0 ❌ ±0 
8 394 runs  ±0  8 285 ✔️ ±0  109 💤 ±0  0 ❌ ±0 

Results for commit 5abaa7e. ± Comparison against base commit dd6ba79.