docs: bump minimal dpdata version to 3.8 in README (#785)

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## Summary by CodeRabbit

- **Documentation**
- Updated Python version compatibility from 3.7 to 3.8 and above in
README

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README.md

@@ -8,7 +8,7 @@
 
 ## Installation
 
-dpdata only supports Python 3.7 and above. You can [setup a conda/pip environment](https://docs.deepmodeling.com/faq/conda.html), and then use one of the following methods to install dpdata:
+dpdata only supports Python 3.8 and above. You can [setup a conda/pip environment](https://docs.deepmodeling.com/faq/conda.html), and then use one of the following methods to install dpdata:
 
 - Install via pip: `pip install dpdata`
 - Install via conda: `conda install -c conda-forge dpdata`