Fix RuboCop error and improve validation logic

- Remove redundant backslash escape in .py regex
- Change validation from AND to OR (more correct logic)
- Improve error message to mention 'readable' file

Author: sbryngelson

packaging/homebrew/mfc.rb

@@ -145,7 +145,7 @@ def install
         fi
 
         # Require a readable Python file (not a directory)
-        if [[ ! "${first_nonflag}" =~ \.py$ ]] || [[ ! -f "${first_nonflag}" ]]; then
+        if [[ ! "${first_nonflag}" =~ .py$ ]] || [[ ! -f "${first_nonflag}" ]]; then
           echo "mfc (Homebrew): first argument must be a readable Python case file."
           echo "Given: ${first_nonflag}"
           echo "Usage: mfc <case.py> [options]"