Remove unsupported flags

Allow-insecure was renamed to allow-unverified in pip v1.5 and then
allow-unverified became unsupported with allow-external in v10.0.0b1.
See https://pip.pypa.io/en/stable/news for details.

install-dev-dependencies.sh

@@ -92,7 +92,7 @@ git submodule init
 git submodule update
 
 # Install Python packages
-pip3 install --user --no-allow-insecure --no-allow-external -r requirements.txt
+pip3 install --user -r requirements.txt
 cd test/rules
 pip3 install --user -r requirements.txt
 cd -