Pin semantic-version dependency to 2.6.0 for the time being

Versions newer than 2.6.0 break Benji. Pin the version until
the cause is found.

setup.py

@@ -61,7 +61,7 @@ def get_version_and_cmdclass(package_path):
         'cerberus>=1.2,<2',
         'pycryptodome>=3.6.1,<4',
         'pyparsing>=2.3.0,<3',
-        'semantic_version>=2.6.0,<3',
+        'semantic_version==2.6.0',
         'dateparser>=0.7.0,<1',
         'structlog>=19.1.0',
         'colorama>=0.4.1,<1',