Deprecate `FS.getbasic` as it is redundant with current `FS.getinfo` behaviour

[althonos]

The FS.getbasic(path) method is supposed to be a shortcut for FS.getinfo(path, namespaces=["basic"]), however every builtin filesystem will provide the basic namespace unconditionally via getinfo. This is even expected in some other places in the code.

In #467 , I updated the FS.getinfo docstring to warn about this behaviour. To go a step further, we should:

• Deprecate the FS.getbasic method, and have it redirect to FS.getinfo instead
• Update FSTestCase so that getinfo is expected to produce the basic namespace unconditionally