refactor: delay import of pandas until needed

[tlambert03]

While working on #86, I noticed that scyjava was greedily importing pandas, even when it wasn't being used. Pandas is somewhat large and can slow down startup; so it would be nice to delay that until we know we need it.

The traditional pattern of try import just to know whether something is importable can be improved:

1. if you want to know whether something is importable, you can use importlib.util.find_spec
2. if you want to know whether someone has already imported something (e.g. to check whether some user object is an instance of some third-party type, without unnecessary import), you can look in sys.modules: sys.modules.get('pandas')

this PR removes the _import_X(required=False) pattern in place of importlib.util.find_spec

[ctrueden]

Awesome! Glad to know there is a way to register these converters more efficiently.

[ctrueden]

This is a great trick, thanks!