python3: install inflection

We are attempting to upgrade all repositories from python2
to python3.  sdbusplus uses inflection and is currently failing
CI on Ifd46a4aeb41fb0e6db2f4310a603b727a5a2c2ff without the python3
version.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Id6031b5c0d0ca052dad53be3ef5eb7ab8264bf3b
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index 73fa510..57a85ef 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -252,6 +252,7 @@
 
 RUN pip install inflection
 RUN pip install pycodestyle
+RUN pip3 install inflection
 RUN pip3 install meson==0.52.0
 
 FROM openbmc-base as openbmc-lcov