Test PyPy3.6 on macos-10.15 instead of macos-latest (which is now macos-11)

.github/workflows/ci.yml

@@ -101,6 +101,9 @@ jobs:
             },
           ]
         exclude:
+          # PyPy 3.6 is EOL and not working on macos-latest (now macos-11)
+          - python-version: pypy-3.6
+            platform: { os: "macos-latest", python-architecture: "x64" }
           # There is no 64-bit pypy on windows for pypy-3.6
           - python-version: pypy-3.6
             platform: { os: "windows-latest", python-architecture: "x64" }
@@ -108,6 +111,14 @@ jobs:
           - python-version: pypy-3.7
             platform: { os: "windows-latest", python-architecture: "x86" }
         include:
+          # PyPy3.6 still runs on macos-10.15
+          - python-version: pypy-3.6
+            platform:
+              {
+                os: "macos-10.15",
+                python-architecture: "x64",
+                rust-target: "x86_64-apple-darwin",
+              }
           # Test minimal supported Rust version
           - rust: 1.41.1
             python-version: 3.9