Support subtypes of basic types when encoding (#428)
The current implementation of `TomlEncoder` doesn't support subtypes of the supported basic types. For example:
```py
from enum import StrEnum
import toml
class ValidStrings(StrEnum):
FOO = "foo"
assert isinstance(ValidStrings.FOO, str) # OK
data = {"foo": ValidStrings.FOO}
print(toml.dumps(data))
```
Output:
```
foo = [ "f", "o", "o",]
```
This pull request changes the implementation to use `isinstance` instead of `get(type(...))` to play nice with inheritence.
Fixes #4181 file changed
