use importlib_metadata rather than pbr to introspect versions at runtime

Use of pbr at runtime is super ugly, as all it does is use pkg_resources to
parse distribution versions and pkg_resources could do the exact same thing. But
they are both slow, because pkg_resources is (by design) slow.

importlib.metadata was added to the stdlib in python 3.8 to fulfill this use
case in a speedy and elegant manner.

FIXME: this entire schema is ugly, not merely at runtime -- use of pbr for
versioning from git is an outdated and stale idea, this should use
setuptools_scm instead which:
- is standard
- supports writing the version to _version.py

guake/__init__.py

@@ -23,9 +23,13 @@
 
 def guake_version():
     # Do not import in the module root to speed up the dbus communication as much as possible
-    import pbr.version
+    # That being said, importlib.metadata is pretty speedy unlike pbr/pkg_resources
+    try:
+        import importlib.metadata as importlib_metadata
+    except ImportError:
+        import importlib_metadata
 
-    return pbr.version.VersionInfo("guake").version_string()
+    return importlib_metadata.version("guake")
 
 
 def vte_version():

requirements.txt

@@ -10,5 +10,5 @@
 # `Pipfile.lock` and then regenerate `requirements*.txt`.
 ################################################################################
 
-pbr
+importlib_metadata; python_version < '3.8'
 typing ; python_version < '3.5'