Make pushsource compatible with attrs version 22.2.0

Since new attrs version release, pub tests have begun failing with the
error "TypeError: keywords must be strings". Attrs have added an "alias"
parameter to each attribute[1], which is used in the attrs's "evolve"
method to construct a new instance. This causes issues with the attribute
"from", which has some special logic implemented in order to be usable
at all[2]. The attribute "from" is generated from "from_", copying all
its parameters. Since "alias" is a new parameter, it isn't copied and
thus its value is set no "None". attrs's "evolve" method uses **kwargs
to create a new instance, and it sets one of the keys in **kwargs to
"None" (from alias). Keys in **kwargs must be strings, which causes the
TypeError. It can be fixed by also setting "alias" in the newly created
"from" attribute. In case of "from_", it has the same value as "name"
parameter ("from_"), which is why we don't want to copy it from the old
attribute, but explicitly set it to "from" (otherwise it would be
"from_"). The change should be backwards compatible with older versions
of attrs.

[1] python-attrs/attrs#950
[2] release-engineering#108

src/pushsource/_impl/model/erratum_fixup.py

@@ -53,6 +53,8 @@ def __init__(self, delegate, attrs_old_to_new):
             # (we do this dynamically to cope with differences between ancient and newer
             # versions of attrs library)
             attr_kwargs = {"name": new_name}
+            if hasattr(old_attr, "alias"):
+                attr_kwargs["alias"] = new_name
             for argname in [
                 "default",
                 "validator",