Delay import of pkg_resources until needed (if ever). (#195)

pkg_resources can be very slow to import.

Alternatively, one could use importlib.resources on Py3.7+, but
then one needs to add version-conditional code for Py3.6 (or a
version-conditional dependency on importlib_resources).

src/python/bezier/__config__.py

@@ -18,8 +18,6 @@
 
 import os
 
-import pkg_resources
-
 
 # Error messages for ``handle_import_error``.
 TEMPLATE = "No module named 'bezier.{}'"  # 3.6, 3.7, 3.8, pypy3
@@ -58,6 +56,8 @@ def modify_path():
     if os.name != "nt":
         return
 
+    import pkg_resources  # pylint: disable=import-outside-toplevel
+
     try:
         extra_dll_dir = pkg_resources.resource_filename("bezier", "extra-dll")
     except ImportError: