Fix the python min version (#711)

Author: qubvel

README.md

@@ -12,7 +12,7 @@ Segmentation based on [PyTorch](https://pytorch.org/).**
 [![PyPI - Downloads](https://img.shields.io/pypi/dm/segmentation-models-pytorch?style=for-the-badge&color=blue)](https://pepy.tech/project/segmentation-models-pytorch) 
 <br>
 [![PyTorch - Version](https://img.shields.io/badge/PYTORCH-1.4+-red?style=for-the-badge&logo=pytorch)](https://pepy.tech/project/segmentation-models-pytorch) 
-[![Python - Version](https://img.shields.io/badge/PYTHON-3.6+-red?style=for-the-badge&logo=python&logoColor=white)](https://pepy.tech/project/segmentation-models-pytorch) 
+[![Python - Version](https://img.shields.io/badge/PYTHON-3.7+-red?style=for-the-badge&logo=python&logoColor=white)](https://pepy.tech/project/segmentation-models-pytorch) 
 
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