Use :const: for constants that had the :attr: role

Note that this intentionally does *not* include some all-caps class
attributes that are not really constants:

- Git.GIT_PYTHON_GIT_EXECUTABLE is set by refresh functions,
  including on subsequent calls, which is an important and
  documented part of what those functions do. (Also, it is set
  automatically from an enviroment variable, which is not constant
  across runs; that it could not even in principle be replaced by a
  specific literal value is a further reason it is not a constant.)

- The Git.USE_SHELL attribute is a more ambiguous case. It is given
  a literal value (False) which it preferably remains. But setting
  it has been documented as something users can do (in the
  changelog, and later in regard to setting it being a deprecated
  operation).

The first of these is decisively not a constant even under a loose
definition, while the second is less clear. I've kept both with the
:attr: role.

git/remote.py

@@ -1177,7 +1177,7 @@ def push(
             individual head which had been updated on the remote side.
 
             If the push contains rejected heads, these will have the
-            :attr:`PushInfo.ERROR` bit set in their flags.
+            :const:`PushInfo.ERROR` bit set in their flags.
 
             If the operation fails completely, the length of the returned
             :class:`PushInfoList` will be 0.

git/util.py

@@ -713,14 +713,14 @@ def update(
         :param op_code:
             Integer allowing to be compared against Operation IDs and stage IDs.
 
-            Stage IDs are :attr:`BEGIN` and :attr:`END`. :attr:`BEGIN` will only be set
-            once for each Operation ID as well as :attr:`END`. It may be that
-            :attr:`BEGIN` and :attr:`END` are set at once in case only one progress
-            message was emitted due to the speed of the operation. Between :attr:`BEGIN`
-            and :attr:`END`, none of these flags will be set.
+            Stage IDs are :const:`BEGIN` and :const:`END`. :const:`BEGIN` will only be
+            set once for each Operation ID as well as :const:`END`. It may be that
+            :const:`BEGIN` and :const:`END` are set at once in case only one progress
+            message was emitted due to the speed of the operation. Between
+            :const:`BEGIN` and :const:`END`, none of these flags will be set.
 
-            Operation IDs are all held within the :attr:`OP_MASK`. Only one Operation ID
-            will be active per call.
+            Operation IDs are all held within the :const:`OP_MASK`. Only one Operation
+            ID will be active per call.
 
         :param cur_count:
             Current absolute count of items.
@@ -730,7 +730,7 @@ def update(
             maximum number of items or if it is (yet) unknown.
 
         :param message:
-            In case of the :attr:`WRITING` operation, it contains the amount of bytes
+            In case of the :const:`WRITING` operation, it contains the amount of bytes
             transferred. It may possibly be used for other purposes as well.
 
         :note: