Draft some eg collapsing helpers

Inside a new `.trionics._beg` and exposed from the subpkg ns in
anticipation of the `strict_exception_groups=False` being removed by
`trio` in py 3.15.

Notes,
- mk an embedded single-exc "extractor" using a `BaseExceptionGroup.exceptions` length
  check, when 1 return the lone child.
- use the above in a new `@acm`, async bc it's most likely to be composed in an
  `async with` tuple-style sequence block, called `collapse_eg()` which
  acts a one line "absorber" for when the above mentioned flag is no
  logner supported by `trio.open_nursery()`.

All untested atm fwiw.. but soon to be used in our test suite(s) likely!

tractor/trionics/__init__.py

@@ -29,3 +29,6 @@ from ._broadcast import (
     BroadcastReceiver as BroadcastReceiver,
     Lagged as Lagged,
 )
+from ._beg import (
+    collapse_eg as collapse_eg,
+)

tractor/trionics/_beg.py

@@ -0,0 +1,59 @@
+# tractor: structured concurrent "actors".
+# Copyright 2018-eternity Tyler Goodlet.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+'''
+`BaseExceptionGroup` related utils and helpers pertaining to
+first-class-`trio` from a historical perspective B)
+
+'''
+from contextlib import (
+    # bontextmanager as cm,
+    asynccontextmanager as acm,
+)
+
+
+def maybe_collapse_eg(
+    beg: BaseExceptionGroup,
+) -> BaseException:
+    '''
+    If the input beg can collapse to a single non-eg sub-exception,
+    return it instead.
+
+    '''
+    if len(excs := beg.exceptions) == 1:
+        return excs[0]
+
+    return beg
+
+
+@acm
+async def collapse_eg():
+    '''
+    If `BaseExceptionGroup` raised in the body scope is
+    "collapse-able" (in the same way that
+    `trio.open_nursery(strict_exception_groups=False)` works) then
+    only raise the lone emedded non-eg in in place.
+
+    '''
+    try:
+        yield
+    except* BaseException as beg:
+        if (
+            exc := maybe_collapse_eg(beg)
+        ) is not beg:
+            raise exc
+
+        raise beg