[python] Bump macOS deployment to 10.15

Fix a problem when bumping to newer macOS runners.  The minimum macOS
deployment target to support `std::filesystem` is 10.15.  (This also
happens to cover all C++17 features that are listed on Apple's website
[[1]].)  This should allow us to use newer GitHub macOS runners (13+)
without issue.

[1]: https://developer.apple.com/xcode/cpp/#c++17

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

lib/Bindings/Python/setup.py

@@ -75,7 +75,11 @@ def run(self):
     cmake_args = [
         "-DCMAKE_BUILD_TYPE=Release",  # not used on MSVC, but no harm
         "-DCMAKE_INSTALL_PREFIX={}".format(os.path.abspath(cmake_install_dir)),
-        "-DCMAKE_OSX_DEPLOYMENT_TARGET=10.14",  # on OSX, min target for C++17
+      # Use the minimum macOS deployment target that supports all c++17
+      # features.
+      #
+      # See: Notes column of https://developer.apple.com/xcode/cpp/#c++17
+        "-DCMAKE_OSX_DEPLOYMENT_TARGET=10.15",
         "-DPython3_EXECUTABLE={}".format(sys.executable.replace("\\", "/")),
         "-DLLVM_ENABLE_PROJECTS=mlir",
         "-DLLVM_EXTERNAL_PROJECTS=circt",