Add pip requirements.txt syntax highlighting

ftdetect/requirements.vim

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+autocmd BufNewFile,BufReadPost */requirements.txt set filetype=requirements

ftplugin/requirements.vim

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+set nospell
+set commentstring=#%s

syntax/requirements.vim

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+if exists('b:current_syntax')
+  finish
+endif
+
+highlight default link requirementsComment Comment
+highlight default link requirementsVersion Identifier
+highlight default link requirementsOperator Operator
+
+syntax region requirementsComment start='^\w*#' end='$' contains=@Spell
+syntax match requirementsVersion '\d\+\.\d\+\.\d\+\w*'
+syntax match requirementsOperator '=='