Read Timeout exception during install_prereq.sh

When running install_prereq.sh the following error occured:
`Collecting pycryptodomex
Downloading https://www.piwheels.org/simple/pycryptodomex/pycryptodomex-3.9.7-cp37-cp37m-linux_armv7l.whl (13.7MB)
4% |█▍ | 614kB 691kB/s eta 0:00:19Exception:..
...
... Read timed out.`

I solved it by adding **--default-timeout=60** to the **pip install** command like this:
`sudo -H python3 -m pip install --upgrade --default-timeout=60 paho-mqtt tornado git+https://github.com/drbild/sslpsk.git pycryptodomex`

After that, it worked fine.

Suggestion: Maybe it is a good idea to add it standard to the install_prereq.sh