Fix loading the gemspec file from rake with Ruby 3.3.

Calculating the current directory name from within the gemspec would
fail.

`__FILE__` was '(eval)' with Ruby < 3.3 and '(eval at
/path/Rakefile:linenumber)' with Ruby >= 3.3.

'(eval)' is treated as the current directory ('.') by File.dirname, etc.
'(eval at /path/Rakefile:linenumber)' is treated as '(eval at /path'.

Rakefile

@@ -4,11 +4,11 @@ require 'rubygems/package_task'
 require 'rake/testtask'
 
 BASE_DIR = File.expand_path(File.dirname(__FILE__))
+GEMSPEC_PATH = File.join(BASE_DIR, 'putty-key.gemspec')
+spec = TOPLEVEL_BINDING.eval(File.read(GEMSPEC_PATH), GEMSPEC_PATH)
 
 task :default => :test
 
-spec = eval(File.read('putty-key.gemspec'))
-
 # Attempt to find the private key and return a spec with added options for
 # signing the gem if found.
 def add_signing_key(spec)

putty-key.gemspec

@@ -1,4 +1,4 @@
-require File.expand_path(File.join('..', 'lib', 'putty', 'key', 'version'), __FILE__)
+require_relative 'lib/putty/key/version'
 
 Gem::Specification.new do |s|
   s.name = 'putty-key'