[Python] Cannot Get Import to Work with Subpackages

The following structure (in Python 3.7) is not allowing me to import class A in module b:

package:
package:
__init__.py
a:
__init__.py
a.py
b:
__init__.py
b.py


The top-level __init__.py is blank. Here are the remaining files:

a

# package/package/a/__init__.py

from .a import A


# package/package/a/a.py

class A:

def __init__(self):
pass


b:

# package/package/b/__init__.py

from .b import B


# package/package/b/b.py

from package.a.a import A

class B:

def __init__(self):
pass


Without doing anything else, on Windows, if I try to run b.py (from within the b folder), I get the following error:

ModuleNotFoundError: No module named 'package.a'


If I add a main.py at the top level:

package:
package:
__init__.py
main.py
a:
__init__.py
a.py
b:
__init__.py
b.py


containing

# package/package/main.py

import a
import b


and run main.py (from within package/package), I get the same error.

If I change b.py to

# package/package/b/b.py

from ..a.a import A

class B:

def __init__(self):
pass


and run b.py (from within the b folder) or main.py (from within package/package), I get the error that

ValueError: attempted relative import beyond top-level package


The python docs make it seem like I should be able to do this though!

Can someone please explain why I am getting these errors? I've seen a couple similar posts to this, but they haven't fixed my problem:

1. Importing Submodules Python 3
2. Python submodule importing correctly in python 3.7 but not 3.6

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