Fix deprecated usage of Kernel#open to open URIs

[Lavode]

Ruby 2.7 deprecated the usage of Kernel#open to open URIs directly,
leading to a deprecation warning and an argument error due to
Kernel#open not handling more than one argument.

You can reproduce the error as follows:

[4] pry(main)> require 'steam-condenser';
[5] pry(main)> ::WebApi.api_key = '*snip*';
[6] pry(main)> SteamId.resolve_vanity_url('morrolan-lz');
/home/michael/.gem/ruby/2.7.2/gems/steam-condenser-1.3.11/lib/steam/community/web_api.rb:130: warning: calling URI.open via Kernel#open is deprecated, call URI.open directly or use URI#open
ArgumentError: extra arguments
from /opt/rubies/ruby-2.7.2/lib/ruby/2.7.0/open-uri.rb:153:in `open_uri'

[Lavode]

Heh, seems URI::open was private up to Ruby 2.7. At the same time, Ruby 2.7's Kernel#open hook by OpenURI seems to not accept arbitrary arguments (for HTTP headers) anymore, throwing the ArgumentError mentioned above.
Seems like a bit of a peculiar way to deprecate this feature, as it does not provide a clean version-agnostic way to have code run on both 2.7 and older versions.

[Arie]

@Lavode I didn't see your PR earlier, but I've made a separate one that I've tested on 2.5.9, 2.7.3 and 3.0.1 (also ran resolve_vanity_url) by wrapping the open call in a separate module based on the RUBY_VERSION.
#28

[koraktor]

Sorry @Lavode for not having the time looking into this PR.

@Arie made a great job fixing the very same problem in #28 and I'll merge this later.

Thank you, nevertheless.