Update Readme to fix setup.cfg

Using [pytest] in setup.cfg with these package versions:

    python 3.7.2
    pytest==4.4.2
    pytest-flake8==1.0.4
    pytest-runner==4.4 

led to this error message:

_Failed: [pytest] section in setup.cfg files is no longer supported, change to [tool:pytest] instead._

I've changed the README from [pytest] to [tool:pytest] .

README.rst

@@ -59,7 +59,7 @@ by adding a ``flake8-max-line-length`` option to your ``setup.cfg``
 or ``tox.ini`` file like this::
 
     # content of setup.cfg
-    [pytest]
+    [tool:pytest]
     flake8-max-line-length = 99
 
 Note that the default will be what naturally comes with `flake8`_
@@ -70,7 +70,7 @@ by adding an ``flake8-ignore`` entry to your ``setup.cfg``
 or ``tox.ini`` file like this::
 
     # content of setup.cfg
-    [pytest]
+    [tool:pytest]
     flake8-ignore = E201 E231
 
 This would globally prevent complaints about two whitespace issues.
@@ -86,7 +86,7 @@ some errors or warnings you can start a flake8-ignore line with
 a glob-pattern and a space-separated list of codes::
 
     # content of setup.cfg
-    [pytest]
+    [tool:pytest]
     flake8-ignore = 
         *.py E201
         doc/conf.py ALL