Obtain package version from SCM

It can be tested with `python setup.py version`.
Note that it's needed to add a tag that conforms semantic versioning.

setup.cfg

@@ -1,7 +1,5 @@
 [metadata]
 name = pEp
-#TODO: automate version pull from hg/git
-version = 2.2.0-RC1
 url = https://pep.foundation
 download_url =
 ext_package = pEp
@@ -33,6 +31,11 @@ python_requires = >= 3.6
 test_suite = tests
 setup_requires =
     setuptools >=38.3.0
+    # To automatically write the version from a SCM.
+    # Note: It fails with tags named as `Release_2.2.0-RC0`, because it is
+    # not conforming to semantic versioning, but it succeed with tags as
+    # `2.2.0-rc0`.
+    setuptools_scm
 install_requires =
     setuptools >=38.3.0
 tests_require =

setup.py

@@ -223,4 +223,7 @@ setup(
     cmdclass={
         'build_ext': BuildExtCommand,
     },
+    # While not using a pyproject.toml, support setuptools_scm setup.cfg usage,
+    # see https://github.com/pypa/setuptools_scm/#setupcfg-usage
+    use_scm_version=True,
 )