Missing dependencies in testsuite documentation. (#1620)

* Missing dependencies in testsuite documentation.

Required package 'pexpect' is not installed as a dependency of
'e3-testsuite', so is has to be explicitly installed.

Neither 'e3-testsuite' nor 'e3-core' depend on it:

$ pip show e3-testsuite e3-core
Name: e3-testsuite
Version: 26.0
Summary: E3 testsuite
Home-page:
Author: AdaCore
Author-email: info@adacore.com
License: GPLv3
Location: /home/ada/opt/alire/venv/lib/python3.10/site-packages
Requires: e3-core
Required-by:
---
Name: e3-core
Version: 22.4.0
Summary: E3 core. Tools and library for building and testing software
Home-page: https://github.com/AdaCore/e3-core
Author: AdaCore
Author-email: info@adacore.com
License: GPLv3
Location: /home/ada/opt/alire/venv/lib/python3.10/site-packages
Requires: colorama, ld, packaging, psutil, python-dateutil, pyyaml, requests, requests-cache, requests-toolbelt, setuptools, stevedore, tqdm
Required-by: e3-testsuite

* Install pip dependencies using requirements.txt