Replace the module alias 'sf' instead of 'F' in pyspark.sql import functions

from pyspark.sql import functions as F

isn’t very Pythonic - it does not follow PEP 8, see Package and Module Names (https://peps.python.org/pep-0008/#package-and-module-names).

Modules should have short, all-lowercase names. Underscores can be used in the module name if it improves
readability. Python packages should also have short, all-lowercase names, although the use of underscores
is discouraged.

Therefore, the module’s alias should follow this. In practice, the uppercase is only used at the module/package
level constants in my experience, see also Constants (https://peps.python.org/pep-0008/#constants).

See also this stackoverflow comment (https://stackoverflow.com/questions/70458086/how-to-correctly-import-pyspark-sql-functions#comment129714058_70458115).