There is an implicit cap due to the C runtime (specifically vcruntime140.dll) allocating fibre local storage on load, which seems like the one you are hitting. However, we discovered this before the first 3.5 release and fixed it.

How are you compiling your packages? Or where are you getting them from?

It's possible that someone else is building and statically linking the C runtime, which will cause this, but distutils (and hence CPython) should not be doing it by default.