GH851 Change use of NoDefault to make it more private

[loicdiridollou]

• Closes Change use of NoDefault to be private and maybe indicate it shouldn't be passed #851
• Tests added: Please use assert_type() to assert the type of any return value

It turns out you can't actually make it fully private with _ at the start of the name as it will raise a lint with PYI (it does not see it used in the file itself although it is used outside the file).

pandas-stubs/_libs/lib.pyi:17:1: PYI047 Private TypeAlias `_NoDefaultDoNotUse` is never used
   |
16 | no_default: Final = _NoDefault.no_default
17 | _NoDefaultDoNotUse: TypeAlias = Literal[_NoDefault.no_default]
   | ^^^^^^^^^^^^^^^^^^ PYI047
18 |
19 | def infer_dtype(value: object, skipna: bool = ...) -> str: ...
   |

Found 1 error. 

[Dr-Irv]

Let's make it private and just put a # noqa: PYI047 on that line.

[Dr-Irv]

thanks @loicdiridollou