Set %__python3 to /usr/bin/python3.6 by default

When %%python3_pkgveresion is set to 3 (i.e. the default), we override
%%__python3 to /usr/bin/python3.6 to ensure that programs are always run
with python3.6, even if /usr/bin/python3 is a symlink to a different
interpreter.

After:

```
$ rpm -E %python3
/usr/bin/python3.6
$ rpm -E %__python3
/usr/bin/python3.6
$ rpm -D 'python3_pkgversion 3.6' -E %__python3
/usr/bin/python3.6
$ rpm -D 'python3_pkgversion 3.8' -E %__python3
/usr/bin/python3.8
$ rpm -D 'python3_pkgversion 3.9' -E %__python3
/usr/bin/python3.9
$ rpm -D 'python3_pkgversion 3.11' -E %__python3
/usr/bin/python3.11
$ rpm -D 'python3_pkgversion 3.11' -E %python3
/usr/bin/python3.11
$ rpm -D 'python3_pkgversion 3.6' -E %python3
/usr/bin/python3.6
$ rpm -E %python3
/usr/bin/python3.6
```

Before:
```
$ rpm -E %python3
/usr/bin/python3
$ rpm -D 'python3_pkgversion 3.8' -E %python3
/usr/bin/python3.8
[...]
```

Relates: https://lists.fedoraproject.org/archives/list/epel-devel@lists.fedoraproject.org/thread/RE3PG72B5AX7NTACPDSBGOWCMN7I3OQJ/

epel-rpm-macros.spec

@@ -1,6 +1,6 @@
 Name:           epel-rpm-macros
 Version:        8
-Release:        33
+Release:        34
 Summary:        Extra Packages for Enterprise Linux RPM macros
 
 License:        GPLv2
@@ -112,6 +112,9 @@ install -Dpm 644 %{SOURCE150} \
 
 
 %changelog
+* Tue Nov 08 2022 Maxwell G <gotmax@e.email> - 8-34
+- Set %%__python3 to /usr/bin/python3.6 by default
+
 * Wed Aug 24 2022 Iñaki Úcar <iucar@fedoraproject.org> - 8-33
 - Backport java_arches macro from Fedora
 

macros.zzz-epel-override

@@ -9,7 +9,23 @@
 # and the default is /usr/libexec/platform-python.
 # Usually, we don't want EPEL packages to use platform-python.
 # Packages that want to do that can explicitly override this.
-%__python3 /usr/bin/python%{_python3_pkgversion_with_dot}
+#
+# When %%python3_pkgveresion is set to 3 (i.e. the default), we override
+# %%__python3 to /usr/bin/python3.6 to ensure that programs are always run with
+# python3.6, even if /usr/bin/python3 is a symlink to a different interpreter.
+#
+# See https://lists.fedoraproject.org/archives/list/epel-devel@lists.fedoraproject.org/thread/RE3PG72B5AX7NTACPDSBGOWCMN7I3OQJ/
+
+%__python3 %{lua:
+local bin = "/usr/bin/python"
+if rpm.expand("%python3_pkgversion") == "3" then
+        bin = bin .. "3.6"
+else
+        bin = bin .. rpm.expand("%{_python3_pkgversion_with_dot}")
+end
+print(bin)
+}
+
 
 # RHEL 8's %%pytest does not work properly on non-default Python versions.
 # Backported from https://src.fedoraproject.org/rpms/python-rpm-macros/c/02536540768555dde6d675935db45df81bcab7bf?branch=rawhide