QGIS per se is not tied to a particular python version, it will currently work with any Python version newer than 3.3. It will also be possible to compile QGIS 3.2 against Python 3.3.
What you probably want to ask is if the QGIS version distributed for Windows is packaged with Python 3.6. QGIS for Windows is produced with OSGeo4W, so the question is, if and when OSGeo4W will update to Python 3.7.
As far as I know, there are no written plans for such an upgrade, but at the other hand, there is also no blocker for preventing it from doing so.
This can be interpreted as there is no schedule for Python 3.7 integration for QGIS on Windows but that does not mean that QGIS is bound to Python 3.6 until a upgrade to QGIS 4. Quite the opposite actually, it's rather likely this upgrade will follow.
If you have a good reason, why you need Python 3.7 because of a particular bug or feature, it's probably best to write to the QGIS developer mailing list and provide details why this upgrade should be prioritized.