:alembic: add C++20 build

.github/workflows/ubuntu.yml

diff --git a/.github/workflows/ubuntu.yml b/.github/workflows/ubuntu.yml
index 711cf10..d6b6540 100644
--- a/.github/workflows/ubuntu.yml
+++ b/.github/workflows/ubuntu.yml
@@ -43,26 +43,6 @@
     - name: test
       run: cd build ; ctest -j 10 --output-on-failure
 
-  gcc_build_cxx20:
-    runs-on: ubuntu-latest
-
-    steps:
-      - uses: actions/checkout@v1
-      - name: install_gcc
-        run: |
-          sudo apt update
-          sudo apt install gcc-10 g++-10
-        shell: bash
-      - name: cmake
-        run: cmake -S . -B build -D CMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STANDARD_REQUIRED=ON
-        env:
-          CC:   gcc-10
-          CXX:  g++-10
-      - name: build
-        run: cmake --build build --parallel 10
-      - name: test
-        run: cd build ; ctest -j 10 --output-on-failure
-
   clang_build_cxx20:
     runs-on: ubuntu-latest
 

test/src/unit-regression2.cpp

diff --git a/test/src/unit-regression2.cpp b/test/src/unit-regression2.cpp
index 1171531..86320f0 100644
--- a/test/src/unit-regression2.cpp
+++ b/test/src/unit-regression2.cpp
@@ -495,7 +495,7 @@
         const char DATA[] = R"("Hello, world!")";
         const auto s = std::as_bytes(std::span(DATA));
         json j = json::parse(s);
-        CHECK(j.dump == "Hello, world!");
+        CHECK(j.dump() == "Hello, world!");
     }
 #endif
 }