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.

docs/conf.py

@@ -9,7 +9,7 @@
 #
 # All configuration values have a default; values that are commented out
 # serve to show the default.
-from pkg_resources import get_distribution
+import tablib
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -49,9 +49,9 @@ copyright = '2019 Jazzband'
 # built documents.
 #
 # The full version, including alpha/beta/rc tags.
-release = get_distribution('tablib').version
+release = tablib.__version__
 # The short X.Y version.
-version = '.'.join(release.split('.')[:2])
+version = '.'.join(tablib.__version__.split('.')[:2])
 # for example take major/minor
 
 # The language for content autogenerated by Sphinx. Refer to documentation