chore(openfeature): pin mixpanel >=5.3.0,<6 for FallbackReason (SDK-126)

The openfeature-provider now dereferences `SelectedVariant.fallback_reason`
(added in SDK-79 / #180). The published mixpanel versions in the current
range (`>=5.1.0,<6` — v5.1.0 and v5.2.0) both predate SDK-79 and do NOT
expose that attribute; installing this provider against either raises
`AttributeError` from `_fallback_details`.

Bump the floor to `>=5.3.0` (the next unreleased minor). CI will fail
until mixpanel 5.3.0 is published with the SDK-79 changes — that failure
is the point: it forces the correct release sequencing so this wrapper
cannot ship against a base version that lacks the field it uses.

Also bump the wrapper version 0.1.0 -> 0.2.0 since fallback-reason
wrapping is a new capability surface.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: tylerjroach

openfeature-provider/pyproject.toml

@@ -4,15 +4,15 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "mixpanel-openfeature"
-version = "0.1.0"
+version = "0.2.0"
 description = "OpenFeature provider for the Mixpanel Python SDK"
 license = "Apache-2.0"
 authors = [
     {name = "Mixpanel, Inc.", email = "dev@mixpanel.com"},
 ]
 requires-python = ">=3.9"
 dependencies = [
-    "mixpanel>=5.1.0,<6",
+    "mixpanel>=5.3.0,<6",
     "openfeature-sdk>=0.7.0",
 ]