fix: Prevent crash with 'gevent' has no attribute 'version_info'

If the `gevent` module is not installed,
but there is a directory called `gevent`
or a `gevent.py` file in the current directory,
then that is already importable and the statment
`if sys.modules['gevent'].version_info < (1, 4):`
fails with
`AttributeError: module 'gevent' has no attribute 'version_info'`

This can happen easily since many libraries like `celery`, `grpc`, `opentracing` etc,
deliver their own file called `gevent.py`.

Signed-off-by: Ferenc Géczi <ferenc.geczi@ibm.com>

src/instana/instrumentation/gevent_inst.py

@@ -37,10 +37,11 @@ def spawn_callback(new_greenlet):
         logger.debug("instrument_gevent: ", exc_info=True)
 
 
-if 'gevent' in sys.modules:
-    if sys.modules['gevent'].version_info < (1, 4):
-        logger.debug("gevent < 1.4 detected.  The Instana package supports gevent versions 1.4 and greater.")
-    else:
-        instrument_gevent()
-else:
+if not 'gevent' in sys.modules:
     logger.debug("Instrumenting gevent: gevent not detected or loaded.  Nothing done.")
+elif not hasattr(sys.modules['gevent'], 'version_info'):
+    logger.debug("gevent module has no 'version_info'. Skipping instrumentation.")
+elif sys.modules['gevent'].version_info < (1, 4):
+    logger.debug("gevent < 1.4 detected.  The Instana package supports gevent versions 1.4 and greater.")
+else:
+    instrument_gevent()