build: pin Sphinx and MarkupSafe to dead-snake-compatible versions

I'm not too bothered about making these bulletproof, since they're only
used for building docs and CI doesn't touch that. They just need to
install cleanly without causing the build to error out.

dev-requirements.txt

@@ -6,6 +6,9 @@ flake8-coding
 flake8-future-import<0.4.6
 flake8-import-order; python_version > '3.3'
 flake8-import-order<=1.18.1; python_version <= '3.3'
+# transitive dependency of Sphinx
+# added f-strings in 2.x, but it's not worth doing a whole suite of version markers
+MarkupSafe<2.0
 pytest<3.3; python_version == '3.3'
 pytest>=4.6,<4.7; python_version != '3.3'
 pytest-vcr==1.0.2; python_version != '3.3'
@@ -14,7 +17,8 @@ PyYAML<5.1; python_version == '3.3'
 PyYAML<5.3; python_version == '3.4'
 requests-mock==1.9.1
 setuptools<40.0; python_version == '3.3'
-sphinx
+# use Sphinx 3.x until dev begins on Sopel 8 and we drop the dead snakes
+sphinx<4.0
 # autoprogram extension added type annotations in 0.1.6
 # such annotations require Python 3.5
 sphinxcontrib-autoprogram<0.1.6; python_version < '3.5'