Description of problem:

Rpm's python3 API has been totally braindamaged all this time but people are only noticing now that it's starting to get used. 

We're changing rpm to return all string data as surrogate-escaped utf-8 python strings everywhere (instead of bytes with unknown encoding that the API doesn't otherwise even accept, see bug 1631292). This makes most rpm-scripts written for python2 just work with python3 too (from the rpm pov).

Most software that has kept python2 compatibility are automatically compatible with the fixed API, but unfortunately python3-only users like mock need fixing for the new behavior.

There's at least one affected place in mock, resulting in a traceback similar to this with the new behavior, unless fixed:

Traceback (most recent call last):
  File "/usr/libexec/mock/mock", line 977, in <module>
    main()
  File "/usr/lib/python3.7/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 766, in main
    run_command(options, args, config_opts, commands, buildroot, state)
  File "/usr/lib/python3.7/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 861, in run_command
    do_rebuild(config_opts, commands, buildroot, options, args)
  File "/usr/lib/python3.7/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 541, in do_rebuild
    post=post_build, clean=clean)
  File "/usr/lib/python3.7/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/libexec/mock/mock", line 482, in rebuild_generic
    ret = cmd(item)
  File "/usr/libexec/mock/mock", line 522, in build
    check=config_opts['check'], spec=options.spec)
  File "/usr/lib/python3.7/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.7/site-packages/mockbuild/backend.py", line 275, in build
    self.installSrpmDeps(rebuilt_srpm)
  File "/usr/lib/python3.7/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.7/site-packages/mockbuild/backend.py", line 198, in installSrpmDeps
    deps.extend(util.getAddtlReqs(hdr, self.more_buildreqs))
  File "/usr/lib/python3.7/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.7/site-packages/mockbuild/util.py", line 400, in getAddtlReqs
    (name, epoch, ver, rel, arch) = getNEVRA(hdr)
  File "/usr/lib/python3.7/site-packages/mockbuild/trace_decorator.py", line 96, in trace
    result = func(*args, **kw)
  File "/usr/lib/python3.7/site-packages/mockbuild/util.py", line 386, in getNEVRA
    return tuple(x.decode() if i != 1 else x for i, x in enumerate(ret))
  File "/usr/lib/python3.7/site-packages/mockbuild/util.py", line 386, in <genexpr>
    return tuple(x.decode() if i != 1 else x for i, x in enumerate(ret))
AttributeError: 'str' object has no attribute 'decode'

As broken versions of the rpm bindings are widely in use, it's recommended that compatibility with both versions is kept for the time being. A possible simple fix is just:
--- util.py.orig	2019-03-28 16:32:22.378591934 +0200
+++ util.py	2019-03-28 16:32:58.009530429 +0200
@@ -342,6 +342,10 @@
     for dummy in yieldSrpmHeaders(srpms, plainRpmOk):
         pass
 
+def b2s(data):
+    if isinstance(data, bytes):
+        return data.decode('utf-8')
+    return data
 
 @traceLog()
 def getNEVRA(hdr):
@@ -354,7 +358,7 @@
     if epoch is None:
         epoch = 0
     ret = (name, epoch, ver, rel, arch)
-    return tuple(x.decode() if i != 1 else x for i, x in enumerate(ret))
+    return tuple(b2s(x) if i != 1 else x for i, x in enumerate(ret))
 

Alternatively, upstream mock version already has a suitable helper added in commit 08b6e4a0eaf7e8317bbef4877302528cb60e6ea5, so the upstream fix would be simply:

--- a/mock/py/mockbuild/util.py
+++ b/mock/py/mockbuild/util.py
@@ -383,7 +383,7 @@ def getNEVRA(hdr):
     if epoch is None:
         epoch = 0
     ret = (name, epoch, ver, rel, arch)
-    return tuple(x.decode() if i != 1 else x for i, x in enumerate(ret))
+    return tuple(_to_text(x) if i != 1 else x for i, x in enumerate(ret))