Problem: test does not run on 2.7 and not very readable

Solution: rewrite in easy_install

doc/getting-started.rst

@@ -452,26 +452,6 @@ to restrict some requirements to i.e. a certain platform or python version:
 
 .. -> src
 
-   >>> prefix = """
-   ... [buildout]
-   ... parts = bobo
-   ... """
-   >>> with open('buildout.cfg', 'w') as f:
-   ...     _ = f.write(prefix)
-   ...     _ = f.write(src)
-
-   >>> import shutil
-   >>> import sys
-   >>> v = sys.version_info
-   >>> shutil.rmtree('eggs')
-   >>> run_buildout('buildout show-picked-versions=true')
-   >>> yup([n for n in ls('eggs') if n.startswith('bobo-2.3.0-')])\
-   ... if v.major >= 3 else\
-   ... yup([n for n in ls('eggs') if n.startswith('bobo-2.2.0-')])
-   >>> yup('bobo==2.3.0' in read('out'))\
-   ... if v.major >= 3 else\
-   ... yup('bobo==2.2.0' in read('out'))
-
 The more common way to pin a version is using a ``versions`` section:
 
 .. code-block:: ini

src/zc/buildout/tests/easy_install.txt

@@ -1524,3 +1524,35 @@ This time, we didn't download from or even query the link server.
 
     >>> zc.buildout.easy_install.install_from_cache(False)
     True
+
+Environment markers
+-------------------
+
+    Specifications can include PEP 496 environment markers.
+
+    >>> _ = get(link_server + 'disable_server_logging')
+    >>> spec = ["demo ==0.1; python_version < '3.9'",
+    ... "demo == 0.2; python_version >= '3.9'"]
+    >>> ws = zc.buildout.easy_install.install(
+    ... spec, dest, links=[link_server], index=link_server+'index/')
+    >>> demo_version = None
+    >>> for egg in ws:
+    ...     if egg.project_name == 'demo':
+    ...         demo_version = egg.version
+    >>> demo_version is not None
+    True
+    >>> awaited = ('0.1'
+    ... if (sys.version_info.major <= 3
+    ... and sys.version_info.minor < 9)
+    ... else '0.2')
+    >>> demo_version == awaited
+    True
+
+    Conflicts properly with version spec.
+
+    >>> ws = zc.buildout.easy_install.install(
+    ... spec, dest, links=[link_server], index=link_server+'index/',
+    ... versions = dict(demo='0.3'))
+    Traceback (most recent call last):
+    ...
+    zc.buildout.easy_install.IncompatibleConstraintError: The requirement ('demo==0...