added fizzbuzz implementation

.gitlab-ci.yml

+image: "python:3.7"
+
+before_script:
+  - pip install -r requirements.txt
+
+test:
+  script:
+    - pytest
+  tags:
+    - docker

fizzbuzz.py

-def fizzbuzz(n:int) -> str:
+def fizzbuzz(n: int) -> str:
     """
     Write a program that prints the numbers from 1 to 100.
     But for multiples of three print "Fizz" instead of the number 
     And for the multiples of five print "Buzz". 
     For numbers which are multiples of both three and five print "FizzBuzz".
     """
-    return ""
-
+    if n % 3 == 0 and n % 5 == 0:
+        return "FizzBuzz"
+    elif n % 3 == 0:
+        return "Fizz"
+    elif n % 5 == 0:
+        return "Buzz"
+    else:
+        return str(n)