chore(openfeature): pin mixpanel >=5.3.0 for FallbackReason (SDK-126)… #411

Job	Run time
lint	10s
test (3.11)	1m 19s
test-openfeature-provider (3.13)	20s
test-openfeature-provider (pypy3.11)	48s
test-openfeature-provider (pypy3.9)	55s
test (3.12)	32s
test (3.13)	27s
test-openfeature-provider (3.11)	31s
test (3.9)	36s
test (pypy3.11)	1m 2s
test-openfeature-provider (3.12)	26s
test-openfeature-provider (3.10)	21s
test (3.10)	29s
test-openfeature-provider (3.9)	26s
test (pypy3.9)	1m 7s
	9m 29s

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