Missing requirement.txt on RtD

build_requirements.py expects requirements.txt to be at the certain
location. Since the documentation is in a subdirectory, it is not where
build_requirements.py would like it to be (missing ./docs)
We build the path of requirements.txt dynamically so it is always in the
same directory than the python script.

docs/.sphinx/build_requirements.py

@@ -1,6 +1,10 @@
 import sys
+import os
+
+import pathlib
+scriptdir=pathlib.Path(__file__).parent.resolve()
+sys.path.append(scriptdir)
 
-sys.path.append('./')
 from custom_conf import *
 
 # The file contains helper functions and the mechanism to build the
@@ -113,7 +117,7 @@ def DeduplicateExtensions(extensionNames: [str]):
     requirements = list(dict.fromkeys(requirements))
     requirements.sort()
 
-    with open(".sphinx/requirements.txt", 'w') as requirements_file:
+    with open(os.path.join(scriptdir, "requirements.txt"), 'w') as requirements_file:
         requirements_file.write(
             "# DO NOT MODIFY THIS FILE DIRECTLY!\n"
             "#\n"