python/cpython

This adds a new function named _PyErr_GetExcInfo() that is a variation of the
original PyErr_GetExcInfo() taking a PyThreadState as its first argument.
That function allows to retrieve the exceptions information of any Python
thread -- not only the current one.

* Add tests for case insensitive check of types and extensions as fallback.
* Add tests for data url with no comma.
* Add tests for read_mime_types.
* Add tests for the mimetypes cli and refactor __main__ code to private function.
* Restore mimetypes.knownfiles value at the end of the test.

Copying property objects results in a TypeError. Steps to reproduce:

```
>>> import copy
>>> obj = property()
>>> copy.copy(obj)
````

This affects both shallow and deep copying.  
My idea for a fix is to add property objects to the list of "atomic" objects in the copy module.
These already include types like functions and type objects.

I also added property objects to the unit tests test_copy_atomic and test_deepcopy_atomic. This is my first PR, and it's highly likely I've made some mistake, so please be kind :)


https://bugs.python.org/issue38293

…6552)

Motivation for this PR (comment from @vstinner in bpo issue):
```
Warning seen o AMD64 Ubuntu Shared 3.x buildbot:
https://buildbot.python.org/all/#/builders/141/builds/2593

test_devnull_output (test.test_a=syncio.test_subprocess.SubprocessThreadedWatcherTests) ...
Warning -- threading_cleanup() failed to cleanup 1 threads (count: 1, dangling: 2)
```
The following implementation details for the new method are TBD:

1) Public vs private

2) Inclusion in `close()`

3) Name

4) Coroutine vs subroutine method

5) *timeout* parameter

If it's a private method, 3, 4, and 5 are significantly less important.

I started with the most minimal implementation that fixes the dangling threads without modifying the regression tests, which I think is particularly important. I typically try to avoid directly modifying existing tests as much as possible unless it's necessary to do so. However, I am open to changing any part of this.


https://bugs.python.org/issue38356

* Add test cases for dataclasses.

* Add test for repr output of field.
* Add test for ValueError to be raised when both default and default_factory are passed.