feat(0845_longest-mountain): add py3 solution

0845_longest-mountain-in-array/README.md

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+You may recall that an array `arr` is a mountain array if and only if:
+
+* `arr.length >= 3`
+* There exists some index `i` (0-indexed) with `0 < i < arr.length - 1` such that:
+  * `arr[0] < arr[1] < ... < arr[i - 1] < arr[i]`
+  * `arr[i] > arr[i + 1] > ... > arr[arr.length - 1]`
+
+Given an integer array `arr`, return _the length of the longest subarray, which is a mountain_. Return `0` if there is no mountain subarray.
+
+Example 1:
+    
+    Input: arr = [2,1,4,7,3,2,5] Output: 5 Explanation: The largest mountain is [1,4,7,3,2] which has length 5. 
+
+Example 2:
+    
+    Input: arr = [2,2,2] Output: 0 Explanation: There is no mountain. 
+
+Constraints:
+
+* `1 <= arr.length <= 104`
+* `0 <= arr[i] <= 104`
+
+Follow up:
+
+* Can you solve it using only one pass?
+* Can you solve it in `O(1)` space?