gh-118164: str(10**10000) hangs if the C _decimal module is missing (#118503)

* Initial stab.

* Test the tentative fix. Hangs "forever" without this change.

* Move the new test to a better spot.

* New comment to explain why _convert_to_str allows any poewr of 10.

* Fixed a comment, and fleshed out an existing test that appeared unfinished.

* Added temporary asserts. Or maybe permanent ;-)

* Update Lib/_pydecimal.py

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

* Remove the new _convert_to_str().

Serhiy and I independently concluded that exact powers of 10
aren't possible in these contexts, so just checking the
string length is sufficient.

* At least for now, add the asserts to the other block too.

* 📜🤖 Added by blurb_it.

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Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>