chore(openfeature): pin mixpanel >=5.3.0 for FallbackReason (SDK-126) #406

Job	Run time
lint	11s
test-openfeature-provider (3.10)	24s
test (pypy3.9)	1m 12s
test (3.13)	24s
test-openfeature-provider (3.9)	35s
test-openfeature-provider (pypy3.11)	44s
test (pypy3.11)	1m 6s
test (3.10)	26s
test (3.9)	36s
test-openfeature-provider (3.11)	27s
test (3.11)	30s
test-openfeature-provider (pypy3.9)	39s
test-openfeature-provider (3.13)	20s
test (3.12)	26s
test-openfeature-provider (3.12)	21s
	8m 21s

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