fix: allow writing generic UHI-compatible histograms (#1128)

* Allow writing generic UHI-compatible histograms

UHI-compatible histograms should have a function variances, but are not
required to have a storage_type. Also, if the histogram is not from hist
or boost_histogram, boost_histogram is None and the storage type check
fails.

* style: pre-commit fixes

---------

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

src/uproot/writing/identify.py

@@ -265,7 +265,8 @@ def to_writable(obj):
             data = obj.values(flow=True)
             fSumw2 = (
                 obj.variances(flow=True)
-                if obj.storage_type == boost_histogram.storage.Weight
+                if boost_histogram is None
+                or obj.storage_type == boost_histogram.storage.Weight
                 else None
             )
 
@@ -293,7 +294,8 @@ def to_writable(obj):
 
             tmp = (
                 obj.variances()
-                if obj.storage_type == boost_histogram.storage.Weight
+                if boost_histogram is None
+                or obj.storage_type == boost_histogram.storage.Weight
                 else None
             )
             fSumw2 = None