fix post install method for py3 in mercurial (spack#6497)

* fix post install method for py3 in mercurial

* Update package.py

* Update package.py

* use print_str instead of manual print detection

* Update package.py

* use only depends_on

* Update package.py

* Update package.py

State py3 compatibility for @4.3: according to setup.py from the 4.3.tar.gz from pypi

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

@@ -39,6 +39,7 @@ class Mercurial(PythonPackage):
         'mercurial.httpclient', 'mercurial.pure'
     ]
 
+    version('4.4.1', '37974a416d1d9525e1375c92025b16d9')
     version('4.1.2', '934c99808bdc8385e074b902d59b0d93')
     version('3.9.1', '3759dd10edb8c1a6dfb8ff0ce82658ce')
     version('3.9',   'e2b355da744e94747daae3a5339d28a0')
@@ -47,7 +48,8 @@ class Mercurial(PythonPackage):
     version('3.8.2', 'c38daa0cbe264fc621dc3bb05933b0b3')
     version('3.8.1', '172a8c588adca12308c2aca16608d7f4')
 
-    depends_on('python@2.6:2.8')
+    depends_on('python@2.6:2.8', when='@:4.2.99')
+    depends_on('python@2.7:2.8,3.5:3.5.999,3.6.2:', when='@4.3:')
     depends_on('py-docutils', type='build')
     depends_on('py-pygments', type=('build', 'run'))
     depends_on('py-certifi',  type=('build', 'run'))
@@ -84,8 +86,8 @@ def configure_certificates(self):
         hgrc_filename = join_path(etc_dir, 'hgrc')
 
         # Use certifi to find the location of the CA certificate
-        certificate = python('-c', 'import certifi; print certifi.where()',
-                             output=str)
+        print_str = self.spec['python'].package.print_string('certifi.where()')
+        certificate = python('-c', 'import certifi; ' + print_str)
 
         if not certificate:
             tty.warn('CA certificate not found. You may not be able to '