Python3 dependency is not obligatory

Some of the CI-Linux-Distros jobs failed because of this. Python3 is not
available by default on Debian:9 nor in Centos:8.

Since we are not currently running the tests in those platforms, and
python3 is just used for testing, we do not want to enforce this project
dependency.

CMakeLists.txt

@@ -91,21 +91,23 @@ if( EXIV2_BUILD_UNIT_TESTS )
 endif()
 
 if( EXIV2_BUILD_SAMPLES )
-    add_test(NAME bashTests
-        WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/tests
-        COMMAND cmake -E env EXIV2_BINDIR=${CMAKE_RUNTIME_OUTPUT_DIRECTORY} ${Python3_EXECUTABLE} runner.py bash_tests/testcases.py --verbose
-    )
-    add_test(NAME versionTests
-        WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/tests
-        COMMAND cmake -E env EXIV2_BINDIR=${CMAKE_RUNTIME_OUTPUT_DIRECTORY} ${Python3_EXECUTABLE} runner.py bash_tests/version_test.py
-    )
-    add_test(NAME pythonTests
-        WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/tests
-        COMMAND cmake -E env EXIV2_BINDIR=${CMAKE_RUNTIME_OUTPUT_DIRECTORY} ${Python3_EXECUTABLE} runner.py
-    )
-
     add_subdirectory( samples )
     get_directory_property(SAMPLES DIRECTORY samples DEFINITION APPLICATIONS)
+
+    if (Python3_Interpreter_FOUND)
+        add_test(NAME bashTests
+            WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/tests
+            COMMAND cmake -E env EXIV2_BINDIR=${CMAKE_RUNTIME_OUTPUT_DIRECTORY} ${Python3_EXECUTABLE} runner.py bash_tests/testcases.py --verbose
+        )
+        add_test(NAME versionTests
+            WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/tests
+            COMMAND cmake -E env EXIV2_BINDIR=${CMAKE_RUNTIME_OUTPUT_DIRECTORY} ${Python3_EXECUTABLE} runner.py bash_tests/version_test.py
+        )
+        add_test(NAME pythonTests
+            WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}/tests
+            COMMAND cmake -E env EXIV2_BINDIR=${CMAKE_RUNTIME_OUTPUT_DIRECTORY} ${Python3_EXECUTABLE} runner.py
+        )
+    endif()
 endif()
 
 if( EXIV2_ENABLE_NLS )

cmake/findDependencies.cmake

@@ -11,7 +11,7 @@ else()
     list(APPEND CMAKE_PREFIX_PATH ${CMAKE_BINARY_DIR})
 endif()
 
-find_package (Python3 COMPONENTS Interpreter REQUIRED)
+find_package (Python3 COMPONENTS Interpreter)
 
 # don't use Frameworks on the Mac (#966)
 if (APPLE)