Do not make assertions about stdlib's doctest.py

The source code in stdlib's doctest.py changed between Python 3.7 and
3.8 sufficiently to break even the cut down assertion we already have.

      {red}      File ".../lib/python3.8/doctest.py", Line NNN, in __run{normal}
    - {red}        compileflags, 1)...{normal}
    + {red}        exec(compile(example.source, filename, "single",{normal}

I get the feeling Python 3.8 points to the first line in a multiline
statement in tracebacks, whereas older Python versions usually pointed
to the last line.

src/zope/testrunner/tests/testrunner-colors.rst

@@ -89,7 +89,7 @@ A failed test run highlights the failures in red:
     Exception raised:
     {red}    Traceback (most recent call last):{normal}
     {red}      File ".../doctest.py", line 1356, in __run{normal}
-    {red}        compileflags, 1)...{normal}
+    {red}        ...{normal}
     {red}      File "<doctest sample2.sampletests_e.eek[0]>", line 1, in ?{normal}
     {red}        f(){normal}
     {red}      File "testrunner-ex/sample2/sampletests_e.py", line 19, in f{normal}
@@ -127,7 +127,7 @@ A failed test run highlights the failures in red:
     Exception raised:
     {red}    Traceback (most recent call last):{normal}
     {red}      File ".../doctest.py", line 1356, in __run{normal}
-    {red}        compileflags, 1)...{normal}
+    {red}        ...{normal}
     {red}      File "<doctest e.rst[1]>", line 1, in ?{normal}
     {red}        f(){normal}
     {red}      File "<doctest e.rst[0]>", line 2, in f{normal}