exporter/prometheus-remote-write: fix test requirements for pypy (#2692)

Author: xrmx

exporter/opentelemetry-exporter-prometheus-remote-write/test-requirements.txt

@@ -1,7 +1,9 @@
 asgiref==3.7.2
 certifi==2024.2.2
 charset-normalizer==3.3.2
-cramjam==2.8.1
+# We can drop this after bumping baseline to pypy-39
+cramjam==2.1.0; platform_python_implementation == "PyPy"
+cramjam==2.8.1; platform_python_implementation != "PyPy"
 Deprecated==1.2.14
 idna==3.7
 importlib-metadata==6.11.0