Condense requirements files down to one

PEP 508 defines a way to specify platform-specific requirements.
Rather than having to manage two different requirements files, let's
crunch them down to one and use that facility.

For more details on how this works, see:
https://www.python.org/dev/peps/pep-0508/

requirements.txt

@@ -1,4 +1,4 @@
-# Evennia dependencies, for Linux/Mac platforms
+# Evennia dependencies
 
 # general
 django >= 2.2.4, < 2.3
@@ -14,3 +14,6 @@ model_mommy
 # testing
 mock >= 1.0.1
 anything
+
+# windows-specific
+pypiwin32;platform_system=="Windows"

setup.py

@@ -10,11 +10,9 @@ OS_WINDOWS = os.name == "nt"
 
 def get_requirements():
     """
-    To update the requirements for Evennia, edit the requirements.txt
-    file, or win_requirements.txt for Windows platforms.
+    To update the requirements for Evennia, edit the requirements.txt file.
     """
-    filename = 'win_requirements.txt' if OS_WINDOWS else 'requirements.txt'
-    with open(filename, 'r') as f:
+    with open('requirements.txt', 'r') as f:
         req_lines = f.readlines()
     reqs = []
     for line in req_lines:

win_requirements.txt

@@ -1,19 +0,0 @@
-# Evennia dependencies, for Windows platform
-
-# windows specific
-pypiwin32
-
-# general
-django >= 2.2.4, < 2.3
-twisted >= 19.2.1, < 20.0.0
-pillow == 5.2.0
-pytz
-future >= 0.15.2
-django-sekizai
-autobahn >= 17.9.3
-inflect
-model_mommy
-
-# testing
-mock >= 1.0.1
-anything