BLD: Stop shipping cythonized files in sdist

* Adds pyproject.toml to support this.

* Bumps minimum Cython version to the version supporting Python 3.8.

Closes pandas-dev#28341

Author: TomAugspurger

MANIFEST.in

@@ -3,14 +3,18 @@ include LICENSE
 include RELEASE.md
 include README.md
 include setup.py
+include pyproject.toml
 
 graft doc
-prune doc/build
-
 graft LICENSES
-
 graft pandas
 
+prune doc/build
+prune asv_bench
+prune ci
+prune conda.recipe
+prune scripts
+
 global-exclude *.bz2
 global-exclude *.csv
 global-exclude *.dta

pyproject.toml

@@ -0,0 +1,13 @@
+[build-system]
+# Minimum requirements for the build system to execute.
+requires = [
+    "setuptools",
+    "wheel",
+    "Cython>=0.29.13",  # Note: sync with setup.py
+    "numpy==1.13.3; python_version=='3.5' and platform_system!='AIX'",
+    "numpy==1.13.3; python_version=='3.6' and platform_system!='AIX'",
+    "numpy==1.14.5; python_version>='3.7' and platform_system!='AIX'",
+    "numpy==1.16.0; python_version=='3.5' and platform_system=='AIX'",
+    "numpy==1.16.0; python_version=='3.6' and platform_system=='AIX'",
+    "numpy==1.16.0; python_version>='3.7' and platform_system=='AIX'",
+]

setup.py

@@ -43,7 +43,7 @@ def is_platform_mac():
 }
 
 
-min_cython_ver = "0.28.2"
+min_cython_ver = "0.29.13"  # Note: sync with pyproject.toml
 try:
     import Cython
 
@@ -367,7 +367,7 @@ def initialize_options(self):
     def run(self):
         if "cython" in cmdclass:
             self.run_command("cython")
-        else:
+        elif "sdist" not in sys.argv:
             # If we are not running cython then
             # compile the extensions correctly
             pyx_files = [(self._pyxfiles, "c"), (self._cpp_pyxfiles, "cpp")]
@@ -532,6 +532,8 @@ def maybe_cythonize(extensions, *args, **kwargs):
         # https://github.com/pandas-dev/pandas/issues/25193
         # TODO: See if this can be removed after pyproject.toml added.
         return extensions
+    elif "sdist" in sys.argv:
+        return extensions
 
     numpy_incl = pkg_resources.resource_filename("numpy", "core/include")
     # TODO: Is this really necessary here?