title: compilation issue in protobuf with nanopb
body: <p>I am using nanopb library</p>
  <p>I seek your suggestion to resolve build issue - I am using protobuf 3.12,nanopb 0.4.2, puython3,gcc . I compiled protoc both c and python but still faced issue while building.</p>
  <pre><code>make 
  protoc -osimple.pb simple.proto   - step done 
  nanopb/examples/simple $ python ../../generator/nanopb_generator.py simple.pb -&gt; following error
  </code></pre>
  <pre><code>$ python3 ../../generator/nanopb_generator.py simple.pb Traceback (most recent call last): File &quot;../../generator/nanopb_generator.py&quot;, line 50, in from .proto import nanopb_pb2 SystemError: Parent module '' not loaded, cannot perform relative import
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last): File &quot;../../generator/nanopb_generator.py&quot;, line 71, in import proto.nanopb_pb2 as nanopb_pb2 File &quot;/home/vagrant/nanopb-0.4.2-linux-x86/generator/proto/nanopb_pb2.py&quot;, line 10, in from google.protobuf import symbol_database as _symbol_database ImportError: cannot import name 'symbol_database'
  </code></pre>
  <p>Please tell me what I am missing. Am I supposed to use Python2 only ?<br />
  Also what features of protobuf are not recommended to use in nanopb ? Does nanopb supports both proto2 and proto3 syntax ?</p>