Fix versions of python packages installed with pip in containers

The gerrit-init container installs python packages with pip. The
versions of the packages were not fixed. This could potentially lead to
compatibility issues and already made it hard to know which version was
used exactly.

This change pins the version of the pymysql and the sqlalchemy packages
to the versions also already used for the tests.

Change-Id: Ic7493ac56af5f0c2aea34af3ff0a2c75486eb59e
diff --git a/container-images/gerrit-init/Dockerfile b/container-images/gerrit-init/Dockerfile
index 0975f68..e6a7b9b 100644
--- a/container-images/gerrit-init/Dockerfile
+++ b/container-images/gerrit-init/Dockerfile
@@ -10,8 +10,8 @@
     apt-get clean && \
     rm -rf /var/lib/apt/lists/* && \
     pip3 install \
-      PyMySQL \
-      sqlalchemy
+      PyMySQL~=0.9.2 \
+      sqlalchemy~=1.3.0
 
 COPY tools/* /var/tools/