correct path to conf.py in makeversionhdr.py
correct path to conf.py in makeversionhdr.py

the py/makeversionhdr.py script was looking for `conf.py` in the
`docs/` directory, but this was relocated in 46e7f8e.  This is used by
the fallback `get_version_info_from_docs_conf` method, which is only
consulted if `git` is unavailable in the build environment.

Closes #791.

adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython