Add missing requirements needed by Sphinx.

requirements/dev.txt

@@ -159,3 +159,12 @@ typing==3.6.2 \
     --hash=sha256:349b1f9c109c84b53ac79ac1d822eaa68fc91d63b321bd9392df15098f746f53 \
     --hash=sha256:63a8255fe7c6269916baa440eb9b6a67139b0b97a01af632e7bd2842e1e02f15 \
     --hash=sha256:d514bd84b284dd3e844f0305ac07511f097e325171f6cc4a20878d11ad771849
+imagesize==0.7.1 \
+    --hash=sha256:6ebdc9e0ad188f9d1b2cdd9bc59cbe42bf931875e829e7a595e6b3abdc05cdfb \
+    --hash=sha256:0ab2c62b87987e3252f89d30b7cedbec12a01af9274af9ffa48108f2c13c6062
+alabaster==0.7.10 \
+    --hash=sha256:2eef172f44e8d301d25aff8068fddd65f767a3f04b5f15b0f4922f113aa1c732 \
+    --hash=sha256:37cdcb9e9954ed60912ebc1ca12a9d12178c26637abdf124e3cde2341c257fe0
+snowballstemmer==1.2.1 \
+    --hash=sha256:9f3bcd3c401c3e862ec0ebe6d2c069ebc012ce142cce209c098ccb5b09136e89 \
+    --hash=sha256:919f26a68b2c17a7634da993d91339e288964f93c274f1343e3bbbe2096e1128