Add BuildRequres for python3-devel

Without this, Python3 RPM builds fail due to missing macros:

  %{__python3}  %{python3_sitelib}/

With this BuildRequires in place, Python3 RPMs build successfully.

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
spec: Make the package architecture independent

Since `gerrymander` doesn't create any binaries, make the spec
architecture indepdendent.

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Remove hardcoded python3 interpreter from __init__.py
Remove hardcoded python3 interpreter from scripts/gerrymander