asn1: Add TODO comment for uses of PyStringMethods::to_cow

[facutuesca]

From #13985 (comment):

Another follow up: can you have us use to_str() on py310+ and to_cow() only on older Pythons? We do 3.11 wheels, so they'll get the perf win.

Since pyo3 already internally uses to_str when calling to_cow in Python 3.10+, we just add a comment to migrate once the minimum Python version is sufficient

Part of #12283

[alex]

I think we're going to have this pattern in several places, let's put it in a function.

[facutuesca]

I think we're going to have this pattern in several places, let's put it in a function.

Changed the PR to only add a TODO comment because pyo3 already internally uses to_str when calling to_cow in Python 3.10+.