Update STM32_gen_PeripheralPins.py for #212

[JohnK1987]

In this PR I did small update of STM32_gen_PeripheralPins.py for protection against using python version older than python3+ and should be solution for #212

Before this PR:

D:\Mbed\MbedCE\MbedCE_test\mbed-os\targets\TARGET_STM\tools>STM32_gen_PeripheralPins.py -m "STM32U575RGTx.xml"

Script version 1.20.5

Checking STM32_open_pin_data repo...
*** git clone https://github.com/STMicroelectronics/STM32_open_pin_data.git ***
*** git clone done

STM32_open_pin_data DB version STM32CubeMX-DB.6.0.90

 * Output directory: D:\Mbed\MbedCE\MbedCE_test\mbed-os\targets\TARGET_STM\tools\TARGET_CUSTOM\TARGET_STM\TARGET_STM32U5\TARGET_STM32U575xG\TARGET_STM32U575RGT
 * Generating PeripheralPins.c and PinNames.h with 'STM32_open_pin_data\mcu\STM32U575RGTx.xml'
 * GPIO file: STM32_open_pin_data\mcu\IP\GPIO-STM32U5x_gpio_v1_0_Modes.xml
Traceback (most recent call last):
  File "D:\Mbed\MbedCE\MbedCE_test\mbed-os\targets\TARGET_STM\tools\STM32_gen_PeripheralPins.py", line 1934, in <module>
    parse_pins()
  File "D:\Mbed\MbedCE\MbedCE_test\mbed-os\targets\TARGET_STM\tools\STM32_gen_PeripheralPins.py", line 1400, in parse_pins
    if "Variant" in s.attributes:
  File "C:\Python27\lib\xml\dom\minidom.py", line 523, in __getitem__
    return self._attrs[attname_or_tuple]
KeyError: 0

After this PR

D:\Mbed\MbedCE\MbedCE_test\mbed-os\targets\TARGET_STM\tools>STM32_gen_PeripheralPins.py -m "STM32U575RGTx.xml"
Traceback (most recent call last):
  File "D:\Mbed\MbedCE\MbedCE_test\mbed-os\targets\TARGET_STM\tools\STM32_gen_PeripheralPins.py", line 30, in <module>
    if sys.version_info[0] < 3: raise Exception("Python 3+ is required (recommended latest one).")
Exception: Python 3+ is required (recommended latest one).

I did real test with Python 2.7

Summary of changes

Impact of changes

Migration actions required

N/A

Documentation

N/A

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@multiplemonomials

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