Bump twisted from 18.9.0 to 19.7.0 in /jmbase

Bumps [twisted](https://github.com/twisted/twisted) from 18.9.0 to 19.7.0.
- [Release notes](https://github.com/twisted/twisted/releases)
- [Changelog](https://github.com/twisted/twisted/blob/trunk/NEWS.rst)
- [Commits](https://github.com/twisted/twisted/compare/twisted-18.9.0...twisted-19.7.0)

Signed-off-by: dependabot[bot] <support@github.com>

jmbase/setup.py

@@ -9,6 +9,6 @@ setup(name='joinmarketbase',
       author_email='',
       license='GPL',
       packages=['jmbase'],
-      install_requires=['future', 'twisted==18.9.0', 'service-identity',
+      install_requires=['future', 'twisted==19.7.0', 'service-identity',
                         'chromalog==1.0.5'],
       zip_safe=False)