fixed python version
@@ -3,7 +3,7 @@ https://github.com/dginst/BitcoinBlockchainTechnology
[![Build Status](https://travis-ci.org/dginst/BitcoinBlockchainTechnology.svg?branch=master)](https://travis-ci.org/dginst/BitcoinBlockchainTechnology) [![Coverage Status](https://coveralls.io/repos/github/dginst/BitcoinBlockchainTechnology/badge.svg?branch=master)](https://coveralls.io/github/dginst/BitcoinBlockchainTechnology?branch=master)
-The `btclib` folder (https://github.com/dginst/BitcoinBlockchainTechnology/tree/master/btclib) contains Python code, namely Python 3.6 or later with type annotation. Most of the material is instrumental in teaching; algorithms are intended for demonstration and not to be used in production environments: they could be broken using side-channel attacks! Moreover, they will probably have major refactorings without care for backward compatibility.
+The `btclib` folder (https://github.com/dginst/BitcoinBlockchainTechnology/tree/master/btclib) contains Python code, namely Python 3.5 or later with type annotation. Most of the material is instrumental in teaching; algorithms are intended for demonstration and not to be used in production environments: they could be broken using side-channel attacks! Moreover, they will probably have major refactorings without care for backward compatibility.
To install `btclib`:
```
