Fix apostrophes as primes

Fixes matplotlib#5864

lib/matplotlib/mathtext.py

@@ -2852,12 +2852,15 @@ def subsuper(self, s, loc, toks):
         sub = None
         super = None
 
-        # Pick all of the apostrophe's out
+        # Pick all of the apostrophes out, including first apostrophes that have
+        # been parsed as characters
         napostrophes = 0
         new_toks = []
         for tok in toks[0]:
             if isinstance(tok, six.string_types) and tok not in ('^', '_'):
                 napostrophes += len(tok)
+            elif isinstance(tok, Char) and tok.c == "'":
+                napostrophes += 1
             else:
                 new_toks.append(tok)
         toks = new_toks
@@ -2912,6 +2915,9 @@ def subsuper(self, s, loc, toks):
                 super = Hlist([])
             for i in range(napostrophes):
                 super.children.extend(self.symbol(s, loc, ['\prime']))
+            # kern() and hpack() needed to get the metrics right after extending
+            super.kern()
+            super.hpack()
 
         # Handle over/under symbols, such as sum or integral
         if self.is_overunder(nucleus):

lib/matplotlib/tests/test_mathtext.py

@@ -43,7 +43,7 @@
     r"$\gamma = \frac{x=\frac{6}{8}}{y} \delta$",
     r'$\limsup_{x\to\infty}$',
     r'$\oint^\infty_0$',
-    r"$f'$",
+    r"$f'\quad f'''(x)\quad ''/\mathrm{yr}$",
     r'$\frac{x_2888}{y}$',
     r"$\sqrt[3]{\frac{X_2}{Y}}=5$",
     r"$\sqrt[5]{\prod^\frac{x}{2\pi^2}_\infty}$",