Fix use of `'U'` mode in Python `open()` function

[amitrdubey]

serial_regression.py errors out while running TestCases/cont_adj_euler/naca0012 test due to the use of 'U' mode in fromlines = open(fromfile, 'U').readlines() in TestCase.py.

'U' mode has been deprecated for a while and removed from latest Python versions.

[pcarruscag]

Thanks, open a PR please :)

[bigfooted]

fixed in #2171