In the Fortran specification, the exponent-letter within a
signed-real-literal-constant is E or D. Therefore, both 3.14159e0 and
3.14159d0 should be classified as a Number.Float.
The current behavior is
>>> print(highlight("3.14159d0", FortranLexer(), HtmlFormatter()))
<div class="highlight"><pre><span></span><span class="mf">3.14159</span><span class="n">d0</span>
</pre></div>
where the only the significand is classified as a Number.Float while
the exponent-letter and exponent are classified as Name.Variable.
After this patch the behavior is
>>> print(highlight("3.14159d0", FortranLexer(), HtmlFormatter()))
<div class="highlight"><pre><span></span><span class="mf">3.14159d0</span>
</pre></div>
which is the expected behavior.