fix scrapy.version_info when SCRAPY_VERSION_FROM_GIT is set

Previously it failed with the following exception with SCRAPY_VERSION_FROM_GIT set:
```
version_info = tuple(int(v) for v in __version__.split('.')[:3])
ValueError: invalid literal for int() with base 10: '0-189-g7814cbf' 
```

After this fix `__version__` parts that can't be converted to int will be left as strings, e.g. `(0, 23, '0-189-g7814cbf')`.