Workaround for rhel8 rpm build (#188)

* Workaround for rhel8 rpm build

Builders of rhel8 rpms doesn't support find_namespace_packages
because of old setuptools. Let's use the same workaround as
in pubtools-pulplib.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

setup.py

@@ -1,4 +1,19 @@
-from setuptools import find_namespace_packages, setup
+from setuptools import setup
+
+try:
+    from setuptools import find_namespace_packages
+except ImportError:
+    # Workaround for RHEL-8 RPM packaging that uses setuptools 39.2
+    # find_namespace_packages is supported since setuptools 40.1
+    # Loosely backported from https://github.com/pypa/setuptools/blob/main/setuptools/discovery.py
+    from setuptools import PackageFinder
+
+    class PEP420PackageFinder(PackageFinder):
+        @staticmethod
+        def _looks_like_package(_path):
+            return True
+
+    find_namespace_packages = PEP420PackageFinder.find
 
 
 def get_description():