[3.6] bpo-31315: Fix an assertion failure in imp.create_dynamic(), when spec.name is not a string. (GH-3257)

[miss-islington]

(cherry picked from commit 9974e1b)

https://bugs.python.org/issue31315

[the-knights-who-say-ni]

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[miss-islington]

@orenmn and @serhiy-storchaka: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks, @serhiy-storchaka!