Following thediscussion about voltage tolerance on the ESP8266 pins I have built a setup for testing the input characteristics using a voltage ramp generated by and arduino in series with a 1.5V AAA battery to ramp up the voltage to 6.5Volts and applied such voltage into the GPIO0 of an ESP01 module in series with a 100 Ohm resistor but haven't noticed the snapback on the input voltage as mentioned in the datasheet.
Later I have found that [ba0sh1] have performed a similar experiment but monitored the current consumption on the input pins. He haven't also noticed the action of the snapback circuit but both our conclusions are the same: The IO pins of the ESP8266 are 5V tolerant.
Next step was to experiment with the board and I have lost a lot of time trying to figure out why to the module got stuck so often until I realize that the parts list of the modules are really kept to a minimum and even the reset line lacks a pull-up resistor! So it is very important to pull up all of the pins (at least with the original firmware).
For doing that I have built a breakout board based on this web page. Now I can move on and load the micropython firmware to experiment with it.