matproj.snl: set pybtex strict mode False (>=0.18)

With the latest version of pybtex (0.18) the [strict mode has been made default](http://pybtex.sourceforge.net/history.html). This means that all former warnings will now cause error exceptions to be raised since the pybtex version has been fixed in pymatgen (see a69eaaeb93191c47aee3d240f41bded0624ac2f7). Hence, the conversion from database dicts to pymatgen objects via ` MPStructureNL.from_dict(snl_dict)`, for instance, now won't result in a usable `Structure` object anymore. This pull request adds a call to `pybtex.errors.set_strict_mode(False)` in `matproj.snl.is_valid_bibtex()` to keep treating pybtex *typos* as warnings in the future (backward-compatibilty)