ci: change python version from 3.6 to 3.8

Problem: python 3.6 is end of life and is now cauing failures
for the ci pipeline.

Change python version tro 3.8 because 3.7 is end of support.

.github/workflows/main.yml

@@ -29,7 +29,7 @@ jobs:
     steps:
     - uses: actions/setup-python@v1
       with:
-        python-version: 3.6
+        python-version: 3.8
     - uses: actions/checkout@v2
       with:
         ref: ${{ github.event.pull_request.head.sha }}
@@ -45,7 +45,7 @@ jobs:
     steps:
     - uses: actions/setup-python@v1
       with:
-        python-version: 3.6
+        python-version: 3.8
     - uses: actions/checkout@v2
       with:
         ref: ${{ github.event.pull_request.head.sha }}

CMakeLists.txt

@@ -70,7 +70,7 @@ endif()
 if(DEFINED ENV{PYTHON})
   set(Python_EXECUTABLE $ENV{PYTHON} CACHE FILEPATH "")
 endif()
-find_package(Python 3.6 COMPONENTS Interpreter Development REQUIRED) # 3.6 or higher
+find_package(Python 3.8 COMPONENTS Interpreter Development REQUIRED) # 3.8 or higher
 execute_process(
   COMMAND "${Python_EXECUTABLE}" -c "import sysconfig as sc; print(sc.get_path('purelib', vars={'base':'${CMAKE_INSTALL_PREFIX}'}))"
   OUTPUT_VARIABLE PYTHON_INSTALL_SITELIB