Caliper depends on python3. (#15260)

* Caliper depends on python3.

The package needs to be told where to find it.

* More flake8 formatting edits.

* Change explicit python3 to spec['python'].command.path

Co-Authored-By: Adam J. Stewart <ajstewart426@gmail.com>

* Removing defunct import for flake8

* Flake8 trailing whitespace warning.

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

var/spack/repos/builtin/packages/caliper/package.py

@@ -69,7 +69,7 @@ class Caliper(CMakePackage):
     depends_on('sosflow@spack', when='@1.0:1.99+sosflow')
 
     depends_on('cmake', type='build')
-    depends_on('python', type='build')
+    depends_on('python@3:', type='build')
 
     # sosflow support not yet in 2.0
     conflicts('+sosflow', '@2.0.0:2.2.99')
@@ -79,6 +79,8 @@ def cmake_args(self):
         spec = self.spec
 
         args = [
+            ('-DPYTHON_EXECUTABLE=%s' %
+                spec['python'].command.path),
             '-DBUILD_TESTING=Off',
             '-DBUILD_DOCS=Off',
             '-DBUILD_SHARED_LIBS=%s' % ('On' if '+shared'  in spec else 'Off'),