Modernize STM32 symbol generator

Previously, the generator parsed datasheet PDF files to map datasheet URLs to part numbers. Now, we instead parse the 'families.xml' file from the STM32 CubeMX MCU database. This file contains a map between part variants and unique 'reference part numbers' that match up with the datasheet PDF file names used in the datasheet URLs. Since this is now a 1-to-1 map, we do not have to parse PDFs anymore.

We keep the datasheet downloader since it is an independently useful utility that may become relevant in case we decide to enrich symbols or footprints with information from these datasheets in the future.

The UI of the generator has changed in that it now allows selection of parts by using a --part-number {PATTERN} shell glob argument via python fnmatch, and the generator now only outputs a single KiCAD library at a time. To generate one library per device family, use the stm32_list_families.py script and call the generator once per family.