[RLlib] Retry pip installs (after waiting n seconds) in install-dependencies.sh (#8354)

ci/travis/install-dependencies.sh

@@ -216,7 +216,20 @@ install_dependencies() {
       # These packages aren't Windows-compatible
       pip_packages+=(blist)  # https://github.com/DanielStutzbach/blist/issues/81#issue-391460716
     fi
-    CC=gcc pip install "${pip_packages[@]}"
+
+    # Try n times; we often encounter OpenSSL.SSL.WantReadError (or others)
+    # that break the entire CI job: Simply retry installation in this case
+    # after n seconds.
+    local status="0";
+    local errmsg="";
+    for i in {1..3};
+    do
+      errmsg=$(CC=gcc pip install "${pip_packages[@]}" 2>&1) && break;
+      status=$errmsg && echo "'pip install ...' failed, will retry after n seconds!" && sleep 30;
+    done
+    if [ "$status" != "0" ]; then
+      echo "${status}" && return 1
+    fi
   fi
 
   if [ "${LINT-}" = 1 ]; then