config: reject minversion if it's a list instead of a single string
config: reject minversion if it's a list instead of a single string

Fixes:

src/_pytest/config/__init__.py:1071: error: Argument 1 to "Version" has incompatible type "Union[str, List[str]]"; expected "str"  [arg-type]

The-Compiler/pytest,nicoddemus/pytest,pytest-dev/pytest,nicoddemus/pytest,markshao/pytest,Akasurde/pytest,RonnyPfannschmidt/pytest,The-Compiler/pytest