Update pattern of package name

c.f.) https://www.python.org/dev/peps/pep-0508/

> ^([A-Z0-9]|[A-Z0-9][A-Z0-9._-]*[A-Z0-9])$

syntax/requirements.vim

@@ -12,7 +12,7 @@ syn case match
 syn region requirementsComment start="[ \t]*#" end="$"
 syn match requirementsCommandOption "\v^\[?--?[a-zA-Z\-]*\]?"
 syn match requirementsVersionSpecifiers "\v(\=\=\=?|\<\=?|\>\=?|\~\=|\!\=)"
-syn match requirementsPackageName "\v^[a-zA-Z][a-zA-Z\-_\.0-9]*"
+syn match requirementsPackageName "\v^([a-zA-Z0-9][a-zA-Z0-9\-_\.]*[a-zA-Z0-9])"
 syn match requirementsVersionControls "\v(git\+?|hg\+|svn\+|bzr\+)\S+"
 
 hi link requirementsComment Comment