Contribution to BUG037

[EmArthur]

• BUG037.
• Name and path of the file in which the fix is proposed (e.g. source/BUG/strings.py).
• Reason for the fix: is_alpha should check if all characters in the string are alphabetic, not if the entire string is alphabetic..
• Proposed changes: write s.isalpha(argument) instead of the previous function