fix: Re-enable activation without codechange (AUTOWRAPT_BOOTSTRAP=instana)

[Ferenc-]

Before commit c222d96, the setup.py contained the following entry point specification:

     entry_points={
             'instana':  ['string = instana:load'],

And entrypoints are a must have when using autowrapt.
Quote from the autowrapt documentation:

list the names of the setuptools entrypoints you wish to activate

This can also be observed in the calls made by autowrapt:

  <frozen importlib._bootstrap>(991)_find_and_load()
  <frozen importlib._bootstrap>(975)_find_and_load_unlocked()
  <frozen importlib._bootstrap>(671)_load_unlocked()
  <frozen importlib._bootstrap_external>(843)exec_module()
  <frozen importlib._bootstrap>(219)_call_with_frames_removed()
  /usr/local/lib/python3.8/site.py(580)<module>()
-> main()
  /usr/local/lib/python3.8/site.py(575)main()
-> execusercustomize()
  /usr/local/lib/python3.8/site-packages/autowrapt/bootstrap.py(46)_execusercustomize()
-> _register_bootstrap_functions()
  /usr/local/lib/python3.8/site-packages/autowrapt/bootstrap.py(27)_register_bootstrap_functions()
-> discover_post_import_hooks(name)
  /usr/local/lib/python3.8/site-packages/wrapt/importer.py(97)discover_post_import_hooks()
-> for entrypoint in pkg_resources.iter_entry_points(group=group):
  /usr/local/lib/python3.8/site-packages/pkg_resources/__init__.py(642)<genexpr>()
-> for entry in dist.get_entry_map(group).values()
> /usr/local/lib/python3.8/site-packages/pkg_resources/__init__.py(2853)get_entry_map()

Which fails to fill up the ep_map, since the entry_points.txt metadata is empty:

   2853             ep_map = self._ep_map = EntryPoint.parse_map(
   2854                 self._get_metadata('entry_points.txt'), self
   2855             )

[pvital]

It looks good to me.