Drop Python 2.7 and 3.4 support

* Update setup.py requires and classifiers
* Drop Python 2.7 and 3.4 from CI
* Update docs dropping 2.7 and 3.4 support
* Fix mock imports and remove tests related to pypi's mock module
* Add py27 and 34 support docs to the sidebar
* Remove usage of six from tmpdir
* Remove six.PY* code blocks
* Remove sys.version_info related code
* Cleanup compat
* Remove obsolete safe_str
* Remove obsolete __unicode__ methods
* Remove compat.PY35 and compat.PY36: not really needed anymore
* Remove unused UNICODE_TYPES
* Remove Jython specific code
* Remove some Python 2 references from docs

Related to #5275

src/_pytest/assertion/__init__.py

@@ -74,10 +74,6 @@ class AssertionState(object):
 
 def install_importhook(config):
     """Try to install the rewrite hook, raise SystemError if it fails."""
-    # Jython has an AST bug that make the assertion rewriting hook malfunction.
-    if sys.platform.startswith("java"):
-        raise SystemError("rewrite not supported")
-
     config._assertstate = AssertionState(config, "rewrite")
     config._assertstate.hook = hook = rewrite.AssertionRewritingHook(config)
     sys.meta_path.insert(0, hook)