Pin mmh3 package version in sonic-slave-stretch docker (sonic-net#14463)

Why I did it
mmh3's new version 3.1.0 breaks pipeline build.
bullseye/buster/jessie pined the version to 2.5.1

How I did it
Pin mmh3's version as other dists.

How to verify it

sonic-slave-stretch/Dockerfile.j2

@@ -376,7 +376,7 @@ RUN pip2 install Pympler==0.8
 RUN pip3 install pyang==2.4.0
 
 # For mgmt-framework build
-RUN pip2 install mmh3
+RUN pip2 install mmh3==2.5.1
 
 # Install dependencies for isc-dhcp-relay build
 RUN apt-get -y build-dep isc-dhcp