requirements: split internal and external requirements in two separate files

MANIFEST.in

 include Makefile
 include requirements.txt
+include requirements-swh.txt
 include version.txt

requirements-swh.txt

+# Add here internal Software Heritage dependencies, one per line.

setup.py

@@ -3,13 +3,13 @@ from setuptools import setup
 
 def parse_requirements():
     requirements = []
-    with open('requirements.txt') as f:
-        for line in f.readlines():
-            line = line.strip()
-            if not line or line.startswith('#'):
-                continue
-            requirements.append(line)
-
+    for reqf in ('requirements.txt', 'requirements-swh.txt'):
+        with open(reqf) as f:
+            for line in f.readlines():
+                line = line.strip()
+                if not line or line.startswith('#'):
+                    continue
+                requirements.append(line)
     return requirements