doc/INSTALL.md: use requirements.txt for pip installs.

We actually need more than just python-bitcoinlib for testing.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

doc/INSTALL.md

@@ -48,7 +48,7 @@ as well:
 For development or running tests, get additional dependencies:
 
     sudo apt-get install -y asciidoc valgrind python3-pip
-    sudo pip3 install python-bitcoinlib
+    sudo pip3 install -r tests/requirements.txt
 
 Clone lightning: