Add support for PEP 508 markers in section condition expressions.

For example: `[versions:python_version <= "3.9"]`.
Fixes https://github.com/buildout/buildout/issues/621

news/621.feature

+Add support for PEP 508 markers in section condition expressions.
+For example: ``[versions:python_version <= "3.9"]``.
+[maurits]

src/zc/buildout/configparser.py

@@ -21,6 +21,15 @@ import re
 import textwrap
 import logging
 
+try:
+    import packaging
+except ImportError:
+    try:
+        from pip._vendor import packaging
+    except ImportError:
+        from pkg_resources import packaging
+
+
 logger = logging.getLogger('zc.buildout')
 
 class Error(Exception):
@@ -179,6 +188,13 @@ def parse(fp, fpname, exp_globals=dict):
                     # un-escape literal # and ; . Do not use a
                     # string-escape decode
                     expr = expression.replace(r'\x23','#').replace(r'\x3b', ';')
+                    try:
+                        # new-style markers as used in pip constraints, e.g.:
+                        # 'python_version < "3.11" and platform_system == "Windows"'
+                        marker = packaging.markers.Marker(expr)
+                        section_condition = marker.evaluate()
+                    except packaging.markers.InvalidMarker:
+                        # old style buildout expression
                         # rebuild a valid Python expression wrapped in a list
                         expr = head + expr + tail
                         # lazily populate context only expression