swan: Install latest version (4.9.0) of pexpect

Jupyterlab git extension uses the pexpect python module as a dependency.
However, the extension only works properly with the latest version
(4.9.0) of pexpect, together with python 3.11, which is the python
version that is used by jupyterlab.

This issue was reported in the repository of the git extension, on
GitHub:
jupyterlab/jupyterlab-git#1261

swan/Dockerfile

@@ -77,7 +77,10 @@ RUN mamba install --yes \
     'jupyter-resource-usage==1.0.1' \
     'jupyterlab-git==0.50.0' \
     # Extensions required by jupyter server
-    'webio-jupyter-extension==0.1.0'
+    'webio-jupyter-extension==0.1.0' \
+    # This version of pexpect is needed for
+    # jupyterlab-git (0.50.0) module to work properly
+    'pexpect==4.9.0'
 
 # Install all of our extensions
 # Ignore (almost all) dependencies because they have already been installed or come from CVMFS