doctest requires code examples have PS1 as ">>> " and PS2 as "... " -- that is, each is three printed characters, followed by a space:

```
$ cat ell_err.py
import doctest

class Foo:
    """Test docstring.

    >>>print("This is a test sentence.")
    ...a test...

    """

doctest.run_docstring_examples(
    Foo(),
    {},
    optionflags=doctest.ELLIPSIS,
)

$ python3.8 --version
Python 3.8.0a3
$ python3.8 ell_err.py
Traceback (most recent call last):
    ...
ValueError: line 3 of the docstring for NoName lacks blank after >>>: '    >>>print("This is a test sentence.")'


$ cat ell_print.py
import doctest

class Foo:
    """Test docstring.

    >>> print("This is a test sentence.")
    ...a test...

    """

doctest.run_docstring_examples(
    Foo(),
    {},
    optionflags=doctest.ELLIPSIS,
)

$ python3.8 ell_print.py
Traceback (most recent call last):
    ...
ValueError: line 4 of the docstring for NoName lacks blank after ...: '    ...a test...'

```

AFAICT, this behavior is consistent across 3.4.10, 3.5.7, 3.6.8, 3.7.3, and 3.8.0a3.


**However**, in this `ell_print.py` above, that "PS2" line isn't actually meant to be a continuation of the 'source' portion of the example; it's meant to be the *output* (the 'want') of the example, with a leading ellipsis to be matched per `doctest.ELLIPSIS` rules.

The regex currently used to look for the 'source' of an example is (https://github.com/python/cpython/blob/4f5a3493b534a95fbb01d593b1ffe320db6b395e/Lib/doctest.py#L583-L586):

```
(?P<source>
    (?:^(?P<indent> [ ]*) >>>    .*)    # PS1 line
    (?:\n           [ ]*  \.\.\. .*)*)  # PS2 lines
\n?
```

Since this pattern is compiled with re.VERBOSE (https://github.com/python/cpython/blob/4f5a3493b534a95fbb01d593b1ffe320db6b395e/Lib/doctest.py#L592), the space-as-fourth-character in PS1/PS2 is not explicitly matched.

I propose changing the regex to:

```
(?P<source>
    (?:^(?P<indent> [ ]*) >>>[ ]    .*)    # PS1 line
    (?:\n           [ ]*  \.\.\.[ ] .*)*)  # PS2 lines
\n?
```

This will then *explicitly* match the trailing space of PS1; it *shouldn't* break any existing doctests, because the parsing code lower down has already been requiring that space to be present in PS1, as shown for `ell_err.py` above.

This will also require an *explicit trailing space* to be present in order for a line starting with three periods to be interpreted as a PS2 line of 'source'; otherwise, it will be treated as part of the 'want'.  I made this change in my local user install of 3.8's doctest.py, and it works as I expect on `ell_print.py`, passing the test:

```
$ python3.8 ell_print.py
$
$ cat ell_wrongprint.py
import doctest

class Foo:
    """Test docstring.

    >>> print("This is a test sentence.")
    ...a foo test...

    """

doctest.run_docstring_examples(
    Foo(),
    {},
    optionflags=doctest.ELLIPSIS,
)

$ python3.8 ell_wrongprint.py
**********************************************************************
File "ell_wrongprint.py", line ?, in NoName
Failed example:
    print("This is a test sentence.")
Expected:
    ...a foo test...
Got:
    This is a test sentence.

```

For completeness, the following piece of regex in the 'want' section (https://github.com/python/cpython/blob/4f5a3493b534a95fbb01d593b1ffe320db6b395e/Lib/doctest.py#L589):

```
    (?![ ]*>>>)  # Not a line starting with PS1
```

should probably also be changed to:

```
    (?![ ]*>>>[ ])  # Not a line starting with PS1
```


I would be happy to put together a PR for this; I would plan to take a ~TDD style approach, implementing a few tests first and then making the regex change.