Fix the handling of modulemd files with unicode characters.

This commit introduces new to_text_type helper method and calls it
for return value of all mmd.dumps() calls. That way, we always
end up with proper unicode string represntation on both python
major versions.

This commit also adds unicode character to description of all
the yaml files we use in the tests so we can be sure MBS can
handle unicode characters properly.

This might be temporary fix, depending on the result of discussion
at https://github.com/fedora-modularity/libmodulemd/issues/184.

tests/test_resolver/test_db.py

@@ -25,6 +25,7 @@ import os
 from datetime import datetime
 from mock import patch, PropertyMock
 import pytest
+from module_build_service.utils import to_text_type
 
 import module_build_service.resolver as mbs_resolver
 from module_build_service import app, db, models, glib, utils, Modulemd
@@ -65,7 +66,7 @@ class TestDBModule:
             time_submitted=datetime(2018, 11, 15, 16, 8, 18),
             time_modified=datetime(2018, 11, 15, 16, 19, 35),
             rebuild_strategy='changed-and-after',
-            modulemd=mmd.dumps()
+            modulemd=to_text_type(mmd.dumps())
         )
         build.buildrequires.append(platform_f300103)
         db.session.add(build)
@@ -105,7 +106,7 @@ class TestDBModule:
             xmd = glib.from_variant_dict(mmd.get_xmd())
             xmd['mbs']['buildrequires'] = {}
             mmd.set_xmd(glib.dict_values(xmd))
-            module.modulemd = mmd.dumps()
+            module.modulemd = to_text_type(mmd.dumps())
             db.session.add(module)
             db.session.commit()
         resolver = mbs_resolver.GenericResolver.create(tests.conf, backend='db')
@@ -134,7 +135,7 @@ class TestDBModule:
             'version': '20180205135154'
         }
         mmd.set_xmd(glib.dict_values(xmd))
-        module.modulemd = mmd.dumps()
+        module.modulemd = to_text_type(mmd.dumps())
         module.name = 'testmodule2'
         module.version = str(mmd.get_version())
         module.koji_tag = 'module-ae2adf69caf0e1b6'