tests: pypi: Rewrite tests using a local HTTP server.

* guix/import/pypi.scm (%pypi-base-url): New variable.
(pypi-fetch): Use it.
* tests/pypi.scm (foo-json): Compute URLs relative to '%local-url'.
(test-json-1, test-json-2, test-source-hash): Remove.
(file-dump): New procedure.
(with-pypi): New macro.
("pypi->guix-package, no wheel")
("pypi->guix-package, wheels")
("pypi->guix-package, no usable requirement file.")
("pypi->guix-package, package name contains \"-\" followed by digits"):
Rewrite using 'with-pypi'.

guix/import/pypi.scm

@@ -55,7 +55,8 @@
   #:use-module (guix packages)
   #:use-module (guix upstream)
   #:use-module ((guix licenses) #:prefix license:)
-  #:export (parse-requires.txt
+  #:export (%pypi-base-url
+            parse-requires.txt
             parse-wheel-metadata
             specification->requirement-name
             guix-package->pypi-name
@@ -67,6 +68,10 @@
 ;; The PyPI API (notice the rhyme) is "documented" at:
 ;; <https://warehouse.readthedocs.io/api-reference/json/>.
 
+(define %pypi-base-url
+  ;; Base URL of the PyPI API.
+  (make-parameter "https://pypi.org/pypi/"))
+
 (define non-empty-string-or-false
   (match-lambda
     ("" #f)
@@ -123,7 +128,7 @@
 
 (define (pypi-fetch name)
   "Return a <pypi-project> record for package NAME, or #f on failure."
-  (and=> (json-fetch (string-append "https://pypi.org/pypi/" name "/json"))
+  (and=> (json-fetch (string-append (%pypi-base-url) name "/json"))
          json->pypi-project))
 
 ;; For packages found on PyPI that lack a source distribution.