Please solve the programming task below using a self-contained code snippet in a markdown code block.

Given a string s and an integer k, return the maximum number of vowel letters in any substring of s with length k.
Vowel letters in English are 'a', 'e', 'i', 'o', and 'u'.
 
Please complete the following python code precisely:
```python
class Solution:
    def maxVowels(self, s: str, k: int) -> int:
```