Get go version from go.mod

No need to update multiple places when bumping the go version.

docs/build.rst

@@ -46,7 +46,7 @@ Run-time dependencies:
 Build-time dependencies:
 
 * ``gcc`` or ``clang``
-* ``go >= 1.20`` (see :file:`go.mod` for go packages used during building)
+* ``go`` >= _build_go_version (see :file:`go.mod` for go packages used during building)
 * ``pkg-config``
 * For building on Linux in addition to the above dependencies you might also
   need to install the following packages, if they are not already installed by

docs/conf.py

@@ -106,8 +106,17 @@ rst_prolog = '''
 '''.replace('VERSION', str_version)
 smartquotes_action = 'qe'  # educate quotes and ellipses but not dashes
 
+def go_version(go_mod_path: str) -> str:  # {{{
+    with open(go_mod_path) as f:
+        for line in f:
+            if line.startswith('go '):
+                return line.strip().split()[1]
+    raise SystemExit(f'No Go version in {go_mod_path}')
+# }}}
+
 string_replacements = {
     '_kitty_install_cmd': 'curl -L https://sw.kovidgoyal.net/kitty/installer.sh | sh /dev/stdin',
+    '_build_go_version': go_version('../go.mod'),
 }