Update build settings

Update excluded folders for flake8
Ensure pip install -e is used in both cases to install ComicTagger
Set required python version to 3.9

.flake8

@@ -1,4 +1,4 @@
 [flake8]
 max-line-length = 120
 extend-ignore = E203, E501, E722
-extend-exclude = venv, scripts
+extend-exclude = venv, scripts, build, dist

Makefile

@@ -78,7 +78,7 @@ $(INSTALL_GUI_STAMP): requirements-GUI.txt
 
 ins: $(PACKAGE_PATH)
 $(PACKAGE_PATH):
-	$(PIP) install .
+	$(PIP) install -e .
 
 dist: CI
 	pyinstaller -y comictagger.spec

setup.py

@@ -48,7 +48,7 @@ setup(
     name="comictagger",
     install_requires=install_requires,
     extras_require=extras_require,
-    python_requires=">=3",
+    python_requires=">=3.9",
     description="A cross-platform GUI/CLI app for writing metadata to comic archives",
     author="ComicTagger team",
     author_email="comictagger@gmail.com",
@@ -68,8 +68,7 @@ setup(
         "License :: OSI Approved :: Apache Software License",
         "Natural Language :: English",
         "Operating System :: OS Independent",
-        "Programming Language :: Python :: 3.5",
-        "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.9",
         "Topic :: Utilities",
         "Topic :: Other/Nonlisted Topic",
         "Topic :: Multimedia :: Graphics",