squash! fixup! fixup! pdb: resume capturing after continue

Store _pytest_capman on the class, for pdbpp's do_debug hack to keep it.

Without this, `debug …` would fail like this:

    /usr/lib/python3.6/cmd.py:217: in onecmd
        return func(arg)
    .venv/lib/python3.6/site-packages/pdb.py:608: in do_debug
        return orig_do_debug(self, arg)
    /usr/lib/python3.6/pdb.py:1099: in do_debug
        sys.call_tracing(p.run, (arg, globals, locals))
    /usr/lib/python3.6/bdb.py:434: in run
        exec(cmd, globals, locals)
    /usr/lib/python3.6/bdb.py:51: in trace_dispatch
        return self.dispatch_line(frame)
    /usr/lib/python3.6/bdb.py:69: in dispatch_line
        self.user_line(frame)
    /usr/lib/python3.6/pdb.py:261: in user_line
        self.interaction(frame, None)
    .venv/lib/python3.6/site-packages/pdb.py:203: in interaction
        self.setup(frame, traceback)
    E   AttributeError: 'PytestPdb' object has no attribute '_pytest_capman'

_pytest/debugging.py

@@ -77,6 +77,8 @@ def set_trace(cls):
             cls._pluginmanager.hook.pytest_enter_pdb(config=cls._config)
 
             class PytestPdb(cls._pdb_cls):
+                _pytest_capman = capman
+
                 def do_continue(self, arg):
                     ret = super(PytestPdb, self).do_continue(arg)
                     if self._pytest_capman:
@@ -103,7 +105,6 @@ def setup(self, f, tb):
                     return ret
 
             _pdb = PytestPdb()
-            _pdb._pytest_capman = capman
         else:
             _pdb = cls._pdb_cls()
         _pdb.set_trace(frame)