BUG: Include "dropna" in Grouper._attributes

All this changes I think is that now `dropna` is included in the
`__repr__` output.
See #46258 for more discussion.

And move dropna up out of the "generated"
attribute section into the "source" attribute
section in the __init__

Author: NickCrews

doc/source/whatsnew/v1.4.3.rst

@@ -24,7 +24,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- Include ``dropna`` in ``__repr__`` of a :class:`Grouper` (:issue:`46754`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/groupby/grouper.py

@@ -263,7 +263,7 @@ class Grouper:
     _gpr_index: Index | None
     _grouper: Index | None
 
-    _attributes: tuple[str, ...] = ("key", "level", "freq", "axis", "sort")
+    _attributes: tuple[str, ...] = ("key", "level", "freq", "axis", "sort", "dropna")
 
     def __new__(cls, *args, **kwargs):
         if kwargs.get("freq") is not None:
@@ -287,6 +287,7 @@ def __init__(
         self.freq = freq
         self.axis = axis
         self.sort = sort
+        self.dropna = dropna
 
         self.grouper = None
         self._gpr_index = None
@@ -295,7 +296,6 @@ def __init__(
         self.binner = None
         self._grouper = None
         self._indexer = None
-        self.dropna = dropna
 
     @final
     @property