Add upper bound to sphinx version

the m2r extension isn't compatible with v3.0; see miyakogi/m2r#55 for example

requirements-dev.txt

@@ -1,6 +1,6 @@
 -r requirements.txt
 m2r>=0.2.0
 mypy>=0.600
-sphinx>=2.0.0
+sphinx>=2.0.0,<3.0.0
 pytest>=4.0.0
 pytest-cov>=2.6.0