internal: Improve version handling for numbagg (#8325)

* internal: Improve version handling for numbagg

Uses the approach in #8316, a bit nicer. Only internal.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update xarray/core/rolling_exp.py

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: max-sixty

xarray/core/rolling_exp.py

@@ -15,9 +15,9 @@
     import numbagg
     from numbagg import move_exp_nanmean, move_exp_nansum
 
-    has_numbagg: Version | None = Version(numbagg.__version__)
+    _NUMBAGG_VERSION: Version | None = Version(numbagg.__version__)
 except ImportError:
-    has_numbagg = None
+    _NUMBAGG_VERSION = None
 
 
 def _get_alpha(
@@ -100,17 +100,17 @@ def __init__(
         window_type: str = "span",
         min_weight: float = 0.0,
     ):
-        if has_numbagg is None:
+        if _NUMBAGG_VERSION is None:
             raise ImportError(
                 "numbagg >= 0.2.1 is required for rolling_exp but currently numbagg is not installed"
             )
-        elif has_numbagg < Version("0.2.1"):
+        elif _NUMBAGG_VERSION < Version("0.2.1"):
             raise ImportError(
-                f"numbagg >= 0.2.1 is required for `rolling_exp` but currently version {has_numbagg} is installed"
+                f"numbagg >= 0.2.1 is required for rolling_exp but currently version {_NUMBAGG_VERSION} is installed"
             )
-        elif has_numbagg < Version("0.3.1") and min_weight > 0:
+        elif _NUMBAGG_VERSION < Version("0.3.1") and min_weight > 0:
             raise ImportError(
-                f"numbagg >= 0.3.1 is required for `min_weight > 0` within `.rolling_exp` but currently version {has_numbagg} is installed"
+                f"numbagg >= 0.3.1 is required for `min_weight > 0` within `.rolling_exp` but currently version {_NUMBAGG_VERSION} is installed"
             )
 
         self.obj: T_DataWithCoords = obj
@@ -211,9 +211,9 @@ def std(self) -> T_DataWithCoords:
         Dimensions without coordinates: x
         """
 
-        if has_numbagg is None or has_numbagg < Version("0.4.0"):
+        if _NUMBAGG_VERSION is None or _NUMBAGG_VERSION < Version("0.4.0"):
             raise ImportError(
-                f"numbagg >= 0.4.0 is required for rolling_exp().std(), currently {has_numbagg} is installed"
+                f"numbagg >= 0.4.0 is required for rolling_exp().std(), currently {_NUMBAGG_VERSION} is installed"
             )
         dim_order = self.obj.dims
 
@@ -243,9 +243,9 @@ def var(self) -> T_DataWithCoords:
         Dimensions without coordinates: x
         """
 
-        if has_numbagg is None or has_numbagg < Version("0.4.0"):
+        if _NUMBAGG_VERSION is None or _NUMBAGG_VERSION < Version("0.4.0"):
             raise ImportError(
-                f"numbagg >= 0.4.0 is required for rolling_exp().var(), currently {has_numbagg} is installed"
+                f"numbagg >= 0.4.0 is required for rolling_exp().var(), currently {_NUMBAGG_VERSION} is installed"
             )
         dim_order = self.obj.dims
 
@@ -275,9 +275,9 @@ def cov(self, other: T_DataWithCoords) -> T_DataWithCoords:
         Dimensions without coordinates: x
         """
 
-        if has_numbagg is None or has_numbagg < Version("0.4.0"):
+        if _NUMBAGG_VERSION is None or _NUMBAGG_VERSION < Version("0.4.0"):
             raise ImportError(
-                f"numbagg >= 0.4.0 is required for rolling_exp().cov(), currently {has_numbagg} is installed"
+                f"numbagg >= 0.4.0 is required for rolling_exp().cov(), currently {_NUMBAGG_VERSION} is installed"
             )
         dim_order = self.obj.dims
 
@@ -308,9 +308,9 @@ def corr(self, other: T_DataWithCoords) -> T_DataWithCoords:
         Dimensions without coordinates: x
         """
 
-        if has_numbagg is None or has_numbagg < Version("0.4.0"):
+        if _NUMBAGG_VERSION is None or _NUMBAGG_VERSION < Version("0.4.0"):
             raise ImportError(
-                f"numbagg >= 0.4.0 is required for rolling_exp().cov(), currently {has_numbagg} is installed"
+                f"numbagg >= 0.4.0 is required for rolling_exp().cov(), currently {_NUMBAGG_VERSION} is installed"
             )
         dim_order = self.obj.dims