Use pip3 in CentOS Stream 8 CI container

There is no 'pip', only 'pip3'

Fixes: c96434d6d288f23a3aa462246ccd6121673b5b69

Signed-off-by: Neal Gompa <ngompa13@gmail.com>

dev/containers/centos8-rpms-py3

@@ -25,7 +25,7 @@ RUN dnf -y install \
     which \
     git
 
-RUN pip install pagure-messages
+RUN pip3 install pagure-messages
 
 RUN cd / \
     && GIT_TRACE=1 GIT_CURL_VERBOSE=1 git clone -b $BRANCH $REPO \