Merge pull request scipy#13217 from rgommers/pin-setuptools-on-16x

REL: add necessary setuptools and numpy version pins in pyproject.toml for 1.6.x branch

Author: tylerjereddy

pyproject.toml

@@ -1,11 +1,12 @@
 [build-system]
 requires = [
     "wheel",
-    "setuptools",
+    "setuptools<=51.0.0",
     "Cython>=0.29.18",
     "numpy==1.16.5; python_version=='3.7'",
     "numpy==1.17.3; python_version=='3.8'",
+    "numpy==1.19.3; python_version=='3.9'",
     # do not pin numpy on future versions of python to avoid incompatible numpy and python versions
-    "numpy; python_version>='3.9'",
+    "numpy; python_version>='3.10'",
     "pybind11>=2.4.3",
 ]