tools/ci.sh: For esp8266 CI install esptool 3.3.1.

Signed-off-by: Damien George <damien@micropython.org>

tools/ci.sh

@@ -149,7 +149,7 @@ function ci_esp32_build {
 # ports/esp8266
 
 function ci_esp8266_setup {
-    sudo pip install pyserial esptool
+    sudo pip install pyserial esptool==3.3.1
     wget https://github.com/jepler/esp-open-sdk/releases/download/2018-06-10/xtensa-lx106-elf-standalone.tar.gz
     zcat xtensa-lx106-elf-standalone.tar.gz | tar x
     # Remove this esptool.py so pip version is used instead