pdb: resume capturing after continue

After `pdb.set_trace()` capturing is turned off.
This patch resumes it after using the `continue` (or `c` / `cont`)
command.

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'

- add pytest_leave_pdb hook
- fixes test_pdb_interaction_capturing_twice: would fail on master now,
  but works here

src/_pytest/hookspec.py

@@ -609,3 +609,13 @@ def pytest_enter_pdb(config):
 
     :param _pytest.config.Config config: pytest config object
     """
+
+
+def pytest_leave_pdb(config):
+    """ called when leaving pdb (e.g. with continue after pdb.set_trace()).
+
+    Can be used by plugins to take special action just after the python
+    debugger leaves interactive mode.
+
+    :param _pytest.config.Config config: pytest config object
+    """