BUG: Pass copy argument to expanddim constructor in concat.
- [x] closes #42501
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Pass the provided copy argument through to the DataFrame constructor when constructing a DataFrame from a Series iterable, as is done with the other DataFrame construction call in the get_result method.

The new test in frame/methods/test_astype.py is as provided by the author of the issue with minor changes.

@TomAugspurger this updates your test `extension/decimal/test_decimal.py:test_astype_dispatches` to expect both frame and series astype calls to retain the context.prec value.  The prior failure was caused by the copying of the DataFrame, and that copy no longer occurs.  The underlying issue with copy and the dtype is unaddressed here.