RPM: Use Python 3 to fix pipe encoding

"setup.py" replaces the shebang line in "leo" with the interpreter that
was used to call "setup.py". Call "setup.py" with python3 so that the
result is consistent with the shebang in git.

For the user, this fixes encoding problems when using pipes, i.e. fixes:

  $ leo Fürst | grep Finsternis
  Prince of Darkness              Fürst der Finsternis

(Using Python 2.x, this led to 'ascii' codec error messages.)

leo-cli.spec

@@ -1,7 +1,7 @@
 %define name leo-cli
 %define version 0.3.4
 %define unmangled_version 0.3.4
-%define release 1
+%define release 2
 
 Summary: leo.org command line tool
 Name: %{name}
@@ -16,6 +16,8 @@ BuildArch: noarch
 Vendor: Johannes Degn <j@degn.de>
 Url: https://github.com/JoiDegn/leo-cli
 
+BuildRequires: python3
+
 %description
 leo-cli is a command line tool which can be used to translate words or phrases
 from several languages to german. It uses the open dictionary dict.leo.org. I
@@ -27,10 +29,10 @@ person.
 %setup -n %{name}-%{unmangled_version}
 
 %build
-python setup.py build
+python3 setup.py build
 
 %install
-python setup.py install -O1 --root=$RPM_BUILD_ROOT --record=INSTALLED_FILES
+python3 setup.py install -O1 --root=$RPM_BUILD_ROOT --record=INSTALLED_FILES
 
 %clean
 rm -rf $RPM_BUILD_ROOT