Fix detection with unittest

ftdetect/requirements.vim

@@ -22,7 +22,7 @@ function! Requirements_matched_filename(filename)
         return 1
     endif
 
-    if a:filename =~# '\v.constraints\.(txt|in)$'
+    if a:filename =~# '\vconstraints\.(txt|in)$'
         return 1
     endif
 

t/test_ftdetect.vim

@@ -5,6 +5,7 @@ describe 'enable: Requirements_matched_filename()'
     it 'to enable file type detection: basic filename'
         Expect Requirements_matched_filename('requirements.txt') to_be_true
         Expect Requirements_matched_filename('requires.txt') to_be_true
+        Expect Requirements_matched_filename('constraints.txt') to_be_true
     end
 
     it 'to enable file type detection: pip-tools filename'