bpo-32528: Make asyncio.CancelledError a BaseException. (GH-13528)

This will address the common mistake many asyncio users make:
an "except Exception" clause breaking Tasks cancellation.

In addition to this change, we stop inheriting asyncio.TimeoutError
and asyncio.InvalidStateError from their concurrent.futures.*
counterparts.  There's no point for these exceptions to share the
inheritance chain.

In 3.9 we'll focus on implementing supervisors and cancel scopes,
which should allow better handling of all exceptions, including
SystemExit and KeyboardInterrupt

Lib/asyncio/sslproto.py

@@ -527,7 +527,9 @@ def data_received(self, data):
 
         try:
             ssldata, appdata = self._sslpipe.feed_ssldata(data)
-        except Exception as e:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as e:
             self._fatal_error(e, 'SSL error in data received')
             return
 
@@ -542,7 +544,9 @@ def data_received(self, data):
                             self._app_protocol, chunk)
                     else:
                         self._app_protocol.data_received(chunk)
-                except Exception as ex:
+                except (SystemExit, KeyboardInterrupt):
+                    raise
+                except BaseException as ex:
                     self._fatal_error(
                         ex, 'application protocol failed to receive SSL data')
                     return
@@ -628,7 +632,9 @@ def _on_handshake_complete(self, handshake_exc):
                 raise handshake_exc
 
             peercert = sslobj.getpeercert()
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             if isinstance(exc, ssl.CertificateError):
                 msg = 'SSL handshake failed on verifying the certificate'
             else:
@@ -691,7 +697,9 @@ def _process_write_backlog(self):
                 # delete it and reduce the outstanding buffer size.
                 del self._write_backlog[0]
                 self._write_buffer_size -= len(data)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             if self._in_handshake:
                 # Exceptions will be re-raised in _on_handshake_complete.
                 self._on_handshake_complete(exc)