Update gcc version to 10 (#857)

Thank you @awvwgk , I will merge this to restore the CI.

Author: perazz

.github/workflows/CI.yml

@@ -22,7 +22,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-11, windows-latest]
-        gcc_v: [9] # Version of GFortran we want to use.
+        gcc_v: [10] # Version of GFortran we want to use.
         include:
         - os: ubuntu-latest
           os-arch: linux-x86_64