patch 9.1.0326: filetype: some requirements files are not recognized

Problem:  filetype: some requirements files are not recognized
Solution: Detect '*-requirements.txt', 'constraints.txt',
          'requirements.in', 'requirements/*.txt' and 'requires/*.txt'
          as requirements filetype, include pip compiler, include
          requirements filetype and syntax plugin
          (Wu, Zhenyu, @raimon49)

closes: #14379

Co-authored-by: raimon <raimon49@hotmail.com>
Signed-off-by: Wu, Zhenyu <wuzhenyu@ustc.edu>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_compiler.vim b/src/testdir/test_compiler.vim
index 26404c6..b7315e1 100644
--- a/src/testdir/test_compiler.vim
+++ b/src/testdir/test_compiler.vim
@@ -71,10 +71,10 @@
   call assert_match('^"compiler ' .. clist .. '$', @:)
 
   call feedkeys(":compiler p\<C-A>\<C-B>\"\<CR>", 'tx')
-  call assert_match('"compiler pandoc pbx perl\( p[a-z]\+\)\+ pylint pyunit', @:)
+  call assert_match('"compiler pandoc pbx perl\( p[a-z_]\+\)\+ pylint pyunit', @:)
 
   call feedkeys(":compiler! p\<C-A>\<C-B>\"\<CR>", 'tx')
-  call assert_match('"compiler! pandoc pbx perl\( p[a-z]\+\)\+ pylint pyunit', @:)
+  call assert_match('"compiler! pandoc pbx perl\( p[a-z_]\+\)\+ pylint pyunit', @:)
 endfunc
 
 func Test_compiler_error()