chore(openfeature): pin mixpanel >=5.3.0 for FallbackReason (SDK-126)… #409

Job	Run time
lint	7s
test-openfeature-provider (3.9)	31s
test-openfeature-provider (3.12)	19s
test-openfeature-provider (3.11)	19s
test-openfeature-provider (pypy3.11)	44s
test (pypy3.9)	1m 20s
test-openfeature-provider (3.13)	22s
test (3.9)	31s
test-openfeature-provider (pypy3.9)	44s
test (3.11)	26s
test (pypy3.11)	1m 8s
test (3.13)	28s
test-openfeature-provider (3.10)	20s
test (3.10)	29s
test (3.12)	27s
	8m 15s

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