fix using url in requirements (ali5h#55)

fixes ali5h#3

examples/tests/requirements.in

@@ -10,3 +10,4 @@ python_dateutil
 
 xgboost
 prometheus_client
+https://github.com/explosion/spacy-models/releases/download/en_core_web_sm-2.3.1/en_core_web_sm-2.3.1.tar.gz#egg=en_core_web_sm==2.3.1

examples/tests/requirements.txt

@@ -1,11 +1,15 @@
 attrs==20.3.0
 azure-core==1.9.0
 azure-storage-blob==12.6.0
+blis==0.7.4
 cachetools==4.1.1
+catalogue==1.0.0
 certifi==2020.11.8
 cffi==1.14.3
 chardet==3.0.4
 cryptography==3.2.1
+cymem==2.0.5
+https://github.com/explosion/spacy-models/releases/download/en_core_web_sm-2.3.1/en_core_web_sm-2.3.1.tar.gz#egg=en_core_web_sm==2.3.1
 google-api-core[grpc]==1.23.0
 google-auth==1.23.0
 google-cloud-language==2.0.0
@@ -17,11 +21,14 @@ isodate==0.6.0
 libcst==0.3.13
 mock==4.0.2
 msrest==0.6.19
+murmurhash==1.0.5
 mypy-extensions==0.4.3
 numpy==1.19.4
 oauthlib==3.1.0
 packaging==20.4
+plac==1.1.3
 pluggy==0.13.1
+preshed==3.0.5
 prometheus-client==0.9.0
 proto-plus==1.11.0
 protobuf==3.14.0
@@ -39,10 +46,15 @@ requests==2.25.0
 rsa==4.6
 scipy==1.5.4
 six==1.15.0
+spacy==2.3.5
+srsly==1.0.5
+thinc==7.4.5
 toml==0.10.2
+tqdm==4.56.0
 typing-extensions==3.7.4.3
 typing-inspect==0.6.0
 urllib3==1.26.2
+wasabi==0.8.2
 xgboost==1.2.0
 
 # The following packages are considered to be unsafe in a requirements file:

src/piptool.py

@@ -3,15 +3,11 @@
 import os
 import re
 import sys
+from collections import OrderedDict
 
 from pip._internal.network.session import PipSession
+from pip._internal.req.constructors import install_req_from_parsed_requirement
 from pip._internal.req.req_file import parse_requirements
-from pip._vendor.packaging.requirements import Requirement
-
-try:
-    from collections import OrderedDict
-except ImportError:
-    OrderedDict = dict
 
 
 def clean_name(name):
@@ -48,7 +44,7 @@ def as_tuple(preq):
     Pulls out the (name: str, version:str, extras:(str)) tuple from
     the pinned ParsedRequirement.
     """
-    req = Requirement(preq.requirement)
+    req = install_req_from_parsed_requirement(preq)
     if not is_pinned_requirement(req, preq.is_editable):
         raise TypeError(
             (