Re-indent the contents of docstrings (#1053)

* Re-indent the contents of docstrings when indentation changes

Keeping the contents of docstrings completely unchanged when
re-indenting (from 2-space intents to 4, for example) can cause
incorrect docstring indentation:

```
class MyClass:
  """Multiline
  class docstring
  """

  def method(self):
    """Multiline
    method docstring
    """
    pass
```
...becomes:
```
class MyClass:
    """Multiline
  class docstring
  """

    def method(self):
        """Multiline
    method docstring
    """
        pass
```

This uses the PEP 257 algorithm for determining docstring indentation,
and adjusts the contents of docstrings to match their new indentation
after `black` is applied.

A small normalization is necessary to `assert_equivalent` because the
trees are technically no longer precisely equivalent -- some constant
strings have changed.  When comparing two ASTs, whitespace after
newlines within constant strings is thus folded into a single space.

Co-authored-by: Luka Zakrajšek <luka@bancek.net>
* Extract the inner `_v` method to decrease complexity

This reduces the cyclomatic complexity to a level that makes flake8 happy.

* Blacken blib2to3's docstring which had an over-indent

Co-authored-by: Luka Zakrajšek <luka@bancek.net>
Co-authored-by: Zsolt Dollenstein <zsol.zsol@gmail.com>