Add defaults to str accessor methods

[hamdanal]

• Work on Do we want to add trivial defaults to the stubs #1292
• Tests added: Please use assert_type() to assert the type of any return value

Added simple default values to the str accessor.

I made a few more changes for things I noticed while adding default values:

• Added a dtype parameter to str.decode
• Fixed the type of str.get key, its documented to accept hashable objects not just int
• Made the table of str.translate covariant as it does not modify the input object
• Marked str.wrap parameters as keyword-only (at runtime they are passed through **kwargs) and fixed their type
• Simplified some redundant overloads and added missing tests for some edge cases

[Dr-Irv]

All looks good, except for a past thing that I should have caught in a past PR review.

[Dr-Irv]

It appears that I missed something when _T_OBJECT was created. We don't want to have Series[object] or Series[type[object]] in the stubs. So can you change the definition of T_OBJECT to be just Series (since we won't know the type) and then change the tests referring to Series[type[object]] to just be Series ?

[Dr-Irv]

thanks @hamdanal