Add conditional version retrieval from setup. (#12853)

When airflow is not installed as package (for example for local
development from sources) there is no package metadata.

Many of our unit tests use the version field and they fail if they
are run within virtual environment where airflow is not installed
as package (for example in IntelliJ this is default setting.

This PR adds fall-back to read airflow version from setup in
case it cannot be read from package metadata.

airflow/version.py

@@ -24,6 +24,13 @@ try:
 except ImportError:
     import importlib_metadata as metadata
 
-version = metadata.version('apache-airflow')
+try:
+    version = metadata.version('apache-airflow')
+except metadata.PackageNotFoundError:
+    import logging
+
+    log = logging.getLogger(__name__)
+    log.warning("Package metadata could not be found. Overriding it with version found in setup.py")
+    from setup import version
 
 del metadata