Fix readmethe docs generation and make file referencing wrong requirements file (#905)

Author: dhoomakethu

.readthedocs.yml

@@ -2,10 +2,10 @@
 formats:
     - epub
     - pdf
-requirements_file: requirements-docs.txt
+requirements_file: requirements.txt
 python:
     extra_requirements:
         - twisted
         - tornado
         - documents
-    version: 3.5
+    version: 3.8

Makefile

@@ -40,7 +40,7 @@ check: install
 
 
 test: install
-	@pip install --upgrade --quiet --requirement=requirements-tests.txt
+	@pip install --upgrade --quiet --requirement=requirements.txt
 	@pip install --upgrade pip --quiet
 	@pytest --cov=pymodbus/ --cov-report term-missing  test
 	@coverage report --fail-under=85 -i
@@ -50,7 +50,7 @@ tox: install
 	@pip install --upgrade --quiet tox && tox
 
 docs: install
-	@pip install --upgrade --quiet --requirement=requirements-docs.txt
+	@pip install --upgrade --quiet --requirement=requirements.txt
 	@cd doc && make clean && make html
 
 publish: install