PEP 0008: Clarify self-contradictory =-spacing rules (#820)

Previously, the PEP contained roughly the following three rules, in sequence:

1. Never use spaces around `=` when used to indicate a parameter default
2. Something unrelated
3. DO use spaces around `=` when used to indicate a parameter default if there is also a parameter annotation present

This commit attempts to clarify this part of the PEP by:
* Combining the first and third rules listed above into a single rule that addresses both annotated and unannotated parameters
* Rephrasing the first rule to indicate that it applies only to unannotated parameters, to eliminate the logical contradiction

pep-0008.txt

@@ -552,19 +552,6 @@ Other Recommendations
       hypot2 = x * x + y * y
       c = (a + b) * (a - b)
 
-- Don't use spaces around the ``=`` sign when used to indicate a
-  keyword argument or a default parameter value.
-
-  Yes::
-
-      def complex(real, imag=0.0):
-          return magic(r=real, i=imag)
-
-  No::
-
-      def complex(real, imag = 0.0):
-          return magic(r = real, i = imag)
-
 - Function annotations should use the normal rules for colons and
   always have spaces around the ``->`` arrow if present.  (See
   `Function Annotations`_ below for more about function annotations.)
@@ -579,9 +566,23 @@ Other Recommendations
       def munge(input:AnyStr): ...
       def munge()->PosInt: ...
 
-- When combining an argument annotation with a default value, use
-  spaces around the ``=`` sign (but only for those arguments that have
-  both an annotation and a default).
+- Don't use spaces around the ``=`` sign when used to indicate a
+  keyword argument, or when used to indicate a default value for an
+  *unannotated* function parameter.
+
+  Yes::
+
+      def complex(real, imag=0.0):
+          return magic(r=real, i=imag)
+
+  No::
+
+      def complex(real, imag = 0.0):
+          return magic(r = real, i = imag)
+
+
+  When combining an argument annotation with a default value, however, do use
+  spaces around the ``=`` sign:
 
   Yes::