bug fix: specify venv version

cyberpanel.sh

@@ -517,7 +517,11 @@ if [[ $DEV == "ON" ]] ; then
       dnf install python3 -y
 			check_return
     fi
-    pip3.6 install virtualenv
+    if [[ $SERVER_OS == "CentOS" ]] ; then
+      pip3.6 install virtualenv==16.7.9
+    else
+      pip3.6 install virtualenv
+    fi
 		check_return
 	fi
 fi
@@ -1130,7 +1134,12 @@ if [[ $PROVIDER == "Alibaba Cloud" ]] ; then
 	pip install setuptools==40.8.0
 fi
 
-pip install virtualenv
+if [[ $SERVER_OS == "CentOS" ]] ; then
+    pip install virtualenv==16.7.9
+  else
+    pip install virtualenv
+fi
+
 virtualenv --system-site-packages /usr/local/CyberPanel
 source /usr/local/CyberPanel/bin/activate
 rm -rf requirements.txt
@@ -1588,4 +1597,4 @@ pip_virtualenv
 
 system_tweak
 
-main_install
+main_install