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On incrTcl: I still cannot reproduce that. On Linux, incrTcl
is typically installed, but it comes as a Tcl package. So it
won't get loaded until somebody does 'package require Itcl'.
On loading that package, I see no change in type of
tcl_version. Grepping the source of Itcl 3.2.1, I find no
occurrence of tcl_version. What specific file contains what
specific access to tcl_version on your system?

On using internal APIs: I see, using GetDoubleFromObj is
probably better. However, it won't change the behaviour at
all: Python accesses the double representation only if it
already knows the Tcl object is a double. In that case,
Tcl_GetDoubleFromObj does exactly the same thing that
_tkinter already does.

On the importance of #518625: It is important for the
following reasons:
a) it allows to round-trip doubles without loss of precision.
    That fixes #721171.
b) it improves type safety, by returning the proper object
types for widget attributes.
c) it improves efficiency, but not requiring expensive
string-object conversions.
It also provides full backwards compatibility, by allowing
applications to set Tkinter.wantobjects=False.

I'll try to come up with a fix for this bug, although I
would really prefer if I could reproduce it.