Speed up imports by removing pkg_resources (hardbyte#1110)

* Add method to calculate load on an interface

* Fix usage of isinstance for calc_load in bus.py

* Include ID length in bus load calculation

* Remove pkg_resources to speed up import time

* Readd pkg_resources but default to importlib to speed up import time

* Resolve handling of importlib entry_points for plugins

* Fall back to ImportError only if importlib is missing

* Remove pkg_resources to speed up import time

* Readd pkg_resources but default to importlib to speed up import time

* Resolve handling of importlib entry_points for plugins

* Fall back to ImportError only if importlib is missing

* Load user-installed interface modules only on demand if importlib is not installed

* Revert "Add method to calculate load on an interface"

This reverts commit 43c0c55

* Revert "Load user-installed interface modules only on demand if importlib is not installed"

This reverts commit 2fed532

* Rename entry_points back to iter_entry_points

Author: wbarnha

can/interfaces/__init__.py

@@ -2,10 +2,6 @@
 Interfaces contain low level implementations that interact with CAN hardware.
 """
 
-import warnings
-from pkg_resources import iter_entry_points
-
-
 # interface_name => (module, classname)
 BACKENDS = {
     "kvaser": ("can.interfaces.kvaser", "KvaserBus"),
@@ -31,11 +27,14 @@
     "neousys": ("can.interfaces.neousys", "NeousysBus"),
 }
 
-BACKENDS.update(
-    {
-        interface.name: (interface.module_name, interface.attrs[0])
-        for interface in iter_entry_points("can.interface")
-    }
-)
+try:
+    from importlib.metadata import entry_points
+    entry = entry_points()
+    if 'can.interface' in entry:
+        BACKENDS.update({interface.name: tuple(interface.value.split(':')) for interface in entry['can.interface']})
+except ImportError:
+    from pkg_resources import iter_entry_points
+    entry = iter_entry_points("can.interface")
+    BACKENDS.update({interface.name: (interface.module_name, interface.attrs[0]) for interface in entry})
 
 VALID_INTERFACES = frozenset(list(BACKENDS.keys()))