docs/requirements.txt: Add minimum version for Sphinx

cameel · cameel · commit 98c4830f2a9f · 2021-06-21T20:54:35.000+02:00
diff --git a/docs/requirements.txt b/docs/requirements.txt
@@ -1,3 +1,6 @@
 sphinx_rtd_theme>=0.3.1
 pygments-lexer-solidity>=0.7.0
 sphinx-a4doc>=1.2.1
+
+# Sphinx 2.1.0 is the oldest version that accepts a lexer class in add_lexer()
+sphinx>=2.1.0
