Stop using pkg_resources

tablib imports pkg_resources in order to find its own version. Importing
pkg_resources is very slow (100ms-250ms is common).

Avoid it by letting setuptools-scm generate a file with the version
instead.

setup.py

@@ -4,7 +4,9 @@ from setuptools import find_packages, setup
 
 setup(
     name='tablib',
-    use_scm_version=True,
+    use_scm_version={
+        'write_to': 'src/tablib/_version.py',
+    },
     setup_requires=['setuptools_scm'],
     description='Format agnostic tabular data library (XLS, JSON, YAML, CSV)',
     long_description=(