[ci] Enable flake8 linting for Python code in util

This isn't as "canonical" as running yapf, but at least the results
shouldn't depend on Python version. The end result is that we run
flake8 on each Python file touched by the change.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/apt-requirements.txt b/apt-requirements.txt
index afb46e8..5a666e9 100644
--- a/apt-requirements.txt
+++ b/apt-requirements.txt
@@ -8,6 +8,7 @@
 build-essential
 clang-format
 curl
+flake8
 flex
 g++
 git
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index a2a7164..ee7841a 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -38,14 +38,15 @@
       yapf --version
       isort --version
       clang-format -version
+      flake8 --version
       echo "PATH=$PATH"
     displayName: Display tool versions
   - bash: |
-      find util -iname '*.py' -print0 \
-        | xargs -0 -n1 $PWD/util/lintpy.py -f
-    # XXX: Python lint checks are disabled until Issue #313 is resolved
-    condition: False
+      fork_origin="$(git merge-base --fork-point origin/master)"
+      git diff --name-only "$fork_origin" "*.py" \
+        | xargs -r util/lintpy.py --tools flake8 -f
     displayName: Run Python lint
+    continueOnError: true
   - bash: |
       make -C hw regs && git diff --exit-code
       if [[ $? != 0 ]]; then