Add doctests to primelib.py (TheAlgorithms#10242)

* Update primelib.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

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

Author: 2 people

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()