doc: fix build with python 3.8

[ upstream commit 878f99d ]

After upgrading to python-3.8.0, a syntax mismatch is revealed:

doc/guides/conf.py:240: SyntaxWarning: "is not" with a literal.
    Did you mean "!="?
    if value is not '':

Removing "is not ''" seems the right thing to do.

A patch may also be needed in the RTD theme package:
https://github.com/readthedocs/sphinx_rtd_theme/commit/a49a812c.diff
(not included in release 0.4.3)

Fixes: 9db3f52 ("doc: generate NIC overview table from ini files")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>

doc/guides/conf.py

@@ -241,7 +241,7 @@ def generate_overview_table(output_filename, table_id, section, table_name, titl
                                                                 ini_filename))
                 continue
 
-            if value is not '':
+            if value:
                 # Get the first letter only.
                 ini_data[ini_filename][name] = value[0]