feat(0504_base-7): add py3 soln

0504_base-7/python3/solution.py

@@ -0,0 +1,11 @@
+class Solution:
+    def convertToBase7(self, num: int) -> str:
+        sign = '-' if num < 0 else ''
+        num = abs(num)
+        b7 = ''
+        
+        while num > 0:
+            b7 = str(num % 7) + b7
+            num //= 7
+        
+        return '0' if b7 == '' else sign + b7