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/