[ci] Remove pinning to setuptools < 50

We did have to pin setuptools due to Debian issues, which are now
resolved and we can remove the pinning. We did the same for Ibex a
while ago and it worked fine since, so we're reasonably confident that
this change will work in OpenTitan as well.

Also remove the explicit installation of six, it's not needed any more
on Ubuntu 18.04.

Signed-off-by: Philipp Wagner <phw@lowrisc.org>
diff --git a/ci/install-package-dependencies.yml b/ci/install-package-dependencies.yml
index ced53f0..66615b9 100644
--- a/ci/install-package-dependencies.yml
+++ b/ci/install-package-dependencies.yml
@@ -57,22 +57,12 @@
       # appropriate bin directory to the PATH
       export PATH=$HOME/.local/bin:$PATH
 
-      # Explicitly installing six is a workaround for pip dependency resolution:
-      # six is already installed as system package with a version below the
-      # required one.  Explicitly installing six through pip gets us a supported
-      # version.
-      #
       # Explicitly updating pip and setuptools is required to have these tools
       # properly parse Python-version metadata, which some packages uses to
       # specify that an older version of a package must be used for a certain
       # Python version. If that information is not read, pip installs the latest
       # version, which then fails to run.
-      python3 -m pip install --user -U pip six
-
-      # There's been a bit of a kerfuffle about setuptools version 50, which
-      # breaks importing distutils on Debian/Ubuntu systems. Make sure we don't
-      # pick it up until the dust has settled and things work again.
-      pip3 install --user -U 'setuptools < 50.0.0'
+      python3 -m pip install --user -U pip setuptools
 
       pip3 install --user -U -r python-requirements.txt