fix PEP 518 support: support markers in build requirements

Author: benoit-pierre

news/5230.bugfix

@@ -0,0 +1 @@
+Fix PEP 518 support: support markers in build requirements.

src/pip/_internal/operations/prepare.py

@@ -53,11 +53,11 @@ def _install_build_reqs(finder, prefix, build_requirements):
     #       that should handle all the isolation and sub-process invocation.
     finder = copy(finder)
     finder.format_control = FormatControl(set(), set([":all:"]))
-    urls = [
-        finder.find_requirement(
-            InstallRequirement.from_line(r), upgrade=False).url
-        for r in build_requirements
-    ]
+    urls = []
+    for req_str in build_requirements:
+        req = InstallRequirement.from_line(req_str)
+        if req.match_markers():
+            urls.append(finder.find_requirement(req, upgrade=False).url)
     args = [
         sys.executable, '-m', 'pip', 'install', '--ignore-installed',
         '--no-user', '--prefix', prefix,

tests/data/packages/pep518-3.0.tar.gz

@@ -1,2 +1,6 @@
 [build-system]
-requires=["simplewheel==2.0", "setuptools", "wheel"]
+requires=[
+	"simplewheel==1.0; python_version < '3'",
+	"simplewheel==2.0; python_version >= '3'",
+	"setuptools", "wheel",
+]