Replace deprecated pipenv lock with pipenv requirements (#7764)

In the versions between what we are currently on in `dependaobt-core`
and the latest versions of `pipenv`, they deprecated and then completely
removed the `pipenv lock [-d]` command in favor of `pipenv requirements
[--dev]` command.

Since our current version of `pipenv` supports the `pipenv requirements`
command, let's proactively switch to it now. That will make the future
upgrade in #7715
easier.

More context:
* pypa/pipenv#5069
* pypa/pipenv#5091
* pypa/pipenv#5198
* pypa/pipenv#5200

Author: jeffwidman

python/lib/dependabot/python/file_updater/pipfile_file_updater.rb

@@ -92,7 +92,7 @@ def generated_requirements_files(type)
 
           # Find any requirement files that list the same dependencies as
           # the (old) Pipfile.lock. Any such files were almost certainly
-          # generated using `pipenv lock -r`
+          # generated using `pipenv requirements`
           requirements_files.select do |req_file|
             deps = []
             req_file.content.scan(regex) { deps << Regexp.last_match }
@@ -237,12 +237,12 @@ def post_process_lockfile(updated_lockfile_content)
 
         def generate_updated_requirements_files
           req_content = run_pipenv_command(
-            "pyenv exec pipenv lock -r"
+            "pyenv exec pipenv requirements"
           )
           File.write("req.txt", req_content)
 
           dev_req_content = run_pipenv_command(
-            "pyenv exec pipenv lock -r -d"
+            "pyenv exec pipenv requirements --dev"
           )
           File.write("dev-req.txt", dev_req_content)
         end

python/spec/dependabot/python/file_updater/pipfile_file_updater_spec.rb

@@ -406,7 +406,7 @@
     context "with a requirements.txt" do
       let(:dependency_files) { [pipfile, lockfile, requirements_file] }
 
-      context "that looks like the output of `pipenv lock -r`" do
+      context "that looks like the output of `pipenv requirements`" do
         let(:pipfile_fixture_name) { "hard_names" }
         let(:lockfile_fixture_name) { "hard_names.lock" }
         let(:requirements_file) do
@@ -471,7 +471,7 @@
         end
       end
 
-      context "that looks like the output of `pipenv lock -r -d`" do
+      context "that looks like the output of `pipenv requirements --dev`" do
         let(:requirements_file) do
           Dependabot::DependencyFile.new(
             name: "req-dev.txt",