Use my fork of vint; fix some lint

dev_requirements.txt

@@ -1,4 +1,6 @@
 flake8==3.8.3
-vim-vint==0.3.21
 flake8-comprehensions==3.2.3
 flake8-ycm>= 0.1.0
+
+# Use fork of vint which is up to date
+git+https://github.com/puremourning/vint

tests/lib/plugin/screendump.vim

@@ -94,7 +94,7 @@ func StopVimInTerminal(buf)
   " In Command-line it's inserted, the CTRL-U removes it again.
   call term_sendkeys(a:buf, "\<C-O>:\<C-U>qa!\<cr>")
 
-  call WaitForAssert({-> assert_equal("finished", term_getstatus(a:buf))})
+  call WaitForAssert({-> assert_equal('finished', term_getstatus(a:buf))})
   only!
 endfunc