Fix exception re-raise in AsyncIOStream.start_tls

[zanieb]

In the previous implementation, the traceback would show "During handling of the above exception, another exception occurred" and include an extra frame.

[tomchristie]

That's interesting, didn't realise there's a different behaviour between those two cases.

How much work is it to demonstrate the before/after of the traceback for this?

[zanieb]

This singular change is a small step in the right direction so it's not quite "fixed" in an example. Maybe I should open a pull request that changes all of these at once? Here are some examples:

WRONG: Displays that an exception occurred while handling an exception

try:
    raise ValueError("1")
except Exception as exc:
    raise ValueError("2")

Traceback (most recent call last):
  File "/Users/mz/dev/prefect/example.py", line 2, in <module>
    raise ValueError("1")
ValueError: 1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/mz/dev/prefect/example.py", line 4, in <module>
    raise ValueError("2")
ValueError: 2

RIGHT: Exception chained to parent

try:
    raise ValueError("1")
except Exception as exc:
    raise ValueError("2") from exc

Traceback (most recent call last):
  File "/Users/mz/dev/prefect/example.py", line 2, in <module>
    raise ValueError("1")
ValueError: 1

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/mz/dev/prefect/example.py", line 4, in <module>
    raise ValueError("2") from exc
ValueError: 2

WRONG: Extra frame included in traceback

try:
    raise ValueError("1")
except Exception as exc:
    raise exc

Traceback (most recent call last):
  File "/Users/mz/dev/prefect/example.py", line 4, in <module>
    raise exc
  File "/Users/mz/dev/prefect/example.py", line 2, in <module>
    raise ValueError("1")
ValueError: 1

RIGHT: No frame added when the exception is re-raised

try:
    raise ValueError("1")
except Exception as exc:
    raise

Traceback (most recent call last):
  File "/Users/mz/dev/prefect/example.py", line 2, in <module>
    raise ValueError("1")
ValueError: 1

[tomchristie]

Maybe I should open a pull request that changes all of these at once?

I think we should deal with these on a case-by-case basis. That'll give us better reviews.

This change looks correct to me, yes. Could you also add the sync and trio cases to this pull request?

[tomchristie]

Hrm, not sure I wasn't able to verify this one...

>>> httpcore.request("GET", "https://142.250.200.36:443")
Traceback (most recent call last):
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_exceptions.py", line 10, in map_exceptions
    yield
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/backends/sync.py", line 62, in start_tls
    raise exc
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/backends/sync.py", line 57, in start_tls
    sock = ssl_context.wrap_socket(
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/ssl.py", line 517, in wrap_socket
    return self.sslsocket_class._create(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/ssl.py", line 1075, in _create
    self.do_handshake()
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/ssl.py", line 1346, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate (_ssl.c:992)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_api.py", line 39, in request
    return pool.request(
           ^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/interfaces.py", line 43, in request
    response = self.handle_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection_pool.py", line 253, in handle_request
    raise exc
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection_pool.py", line 237, in handle_request
    response = connection.handle_request(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 86, in handle_request
    raise exc
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 63, in handle_request
    stream = self._connect(request)
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 141, in _connect
    stream = stream.start_tls(**kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/backends/sync.py", line 54, in start_tls
    with map_exceptions(exc_map):
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/contextlib.py", line 155, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_exceptions.py", line 14, in map_exceptions
    raise to_exc(exc) from exc
httpcore.ConnectError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate (_ssl.c:992)

Let's start with #678 which does look verifiably correct to me, and then go from there.