apply unary spacing to all relational operators

Author: OdileVidrine

lib/matplotlib/_mathtext.py

@@ -2285,7 +2285,8 @@ def symbol(self, s: str, loc: int,
             if (self._in_subscript_or_superscript or (
                     c in self._binary_operators and (
                     len(s[:loc].split()) == 0 or prev_char == '{' or
-                    prev_char in self._left_delims or prev_char == '='))):
+                    prev_char in self._left_delims or 
+                    prev_char in self._relation_symbols))):
                 return [char]
             else:
                 return [Hlist([self._make_space(0.2),

lib/matplotlib/tests/test_mathtext.py

@@ -124,7 +124,7 @@
     r'$,$ $.$ $1{,}234{, }567{ , }890$ and $1,234,567,890$',  # github issue 5799
     r'$\left(X\right)_{a}^{b}$',  # github issue 7615
     r'$\dfrac{\$100.00}{y}$',  # github issue #1888
-    r'$a=-b-c$' # github issue #28180
+    r'$a=-b-c$'  # github issue #28180
 ]
 # 'svgastext' tests switch svg output to embed text as text (rather than as
 # paths).