TST: Move all references to python version in Cygwin CI to single file.

Author: DWesl

.github/workflows/cygwin.yml

@@ -64,4 +64,4 @@ jobs:
           dash -c "/usr/bin/python3.8 -m pip show numpy"
           dash -c "/usr/bin/python3.8 -m pip show -f numpy | grep .dll"
           dash -c "/bin/tr -d '\r' <tools/list_installed_dll_dependencies_cygwin.sh >list_dlls_unix.sh"
-          dash "list_dlls_unix.sh"
+          dash "list_dlls_unix.sh" 3.8

tools/list_installed_dll_dependencies_cygwin.sh

@@ -11,7 +11,7 @@
 # Cygwin-specific, but the rest should work on most platforms with
 # /bin/sh
 
-py_ver=3.7
+py_ver=${1}
 site_packages=$(python${py_ver} -m pip show numpy | \
 		    grep Location | cut -d " " -f 2 -);
 dll_list=$(for name in $(python${py_ver} -m pip show -f numpy | \