@BrenBarn's answer says it all, but if you're like me it might take a while to understand. Here's my case and how @BrenBarn's answer applies to it, perhaps it will help you.
The case
package/
__init__.py
subpackage1/
__init__.py
moduleX.py
moduleA.py
Using our familiar example, and add to it that moduleX.py has a relative import to ..moduleA. Given that I tried writing a test script in the subpackage1 directory that imported moduleX, but then got the dreaded error described by the OP.
Solution
Move test script to the same level as package and import package.subpackage1.moduleX
Explanation
As explained, relative imports are made relative to the current name. When my test script imports moduleX from the same directory, then module name inside moduleX is moduleX. When it encounters a relative import the interpreter can't back up the package hierarchy because it's already at the top
When I import moduleX from above, then name inside moduleX is package.subpackage1.moduleX and the relative import can be found