Add doctests to primelib.py (#10242)

* Update primelib.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: cornbread-eater

maths/primelib.py

@@ -574,6 +574,11 @@ def fib(n):
     """
     input: positive integer 'n'
     returns the n-th fibonacci term , indexing by 0
+
+    >>> fib(5)
+    8
+    >>> fib(99)
+    354224848179261915075
     """
 
     # precondition
@@ -589,3 +594,9 @@ def fib(n):
         fib1 = tmp
 
     return ans
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()