Update supported Python versions

Remove references to, and handling of, Python 2.7 in our project scaffolding:
- updated python_requires in setup.py to state our intent to support
  Python 3.6 and above (but not Python 4, yet)
- Drop no longer required dependencies in setup.py, and requirements-*.txt
  (further refinement of requirements files will be handled in theupdateframework#1161)
- Remove Python 2.7 from our tox environments

Signed-off-by: Joshua Lock <[email protected]>

requirements-pinned.txt

@@ -1,15 +1,11 @@
 certifi==2020.12.5        # via requests
 cffi==1.14.5              # via cryptography, pynacl
 chardet==4.0.0            # via requests
-cryptography==3.4.6       ; python_version >= '3' # via securesystemslib
-cryptography==3.3.2       ; python_version < '3' # via securesystemslib
-enum34==1.1.10            ; python_version < '3' # via cryptography
+cryptography==3.4.6       # via securesystemslib
 idna==2.10                # via requests
-ipaddress==1.0.23         ; python_version < '3' # via cryptography
 pycparser==2.20           # via cffi
 pynacl==1.4.0             # via securesystemslib
 requests==2.25.1
 securesystemslib[crypto,pynacl]==0.20.0
 six==1.15.0
-subprocess32==3.5.4       ; python_version < '3' # via securesystemslib
 urllib3==1.26.3           # via requests

requirements-test.txt

@@ -3,13 +3,10 @@
 # pinned tuf runtime dependencies (should auto-update and -trigger ci/cd)
 -r requirements-pinned.txt
 
-# test runtime dependencies  (see 'tests_require' field in setup.py)
-mock; python_version < "3.3"
-
 # tuf.api tests use python-dateutil
 python-dateutil
 
 # additional test tools for linting and coverage measurement
 coverage
 pylint
-bandit; python_version >= "3.5"
+bandit

setup.py

@@ -97,8 +97,6 @@
     'Operating System :: MacOS :: MacOS X',
     'Operating System :: Microsoft :: Windows',
     'Programming Language :: Python',
-    'Programming Language :: Python :: 2',
-    'Programming Language :: Python :: 2.7',
     'Programming Language :: Python :: 3',
     'Programming Language :: Python :: 3.6',
     'Programming Language :: Python :: 3.7',
@@ -112,15 +110,12 @@
     'Source': 'https://github.com/theupdateframework/tuf',
     'Issues': 'https://github.com/theupdateframework/tuf/issues'
   },
-  python_requires=">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*, <4",
+  python_requires="~=3.6",
   install_requires = [
     'requests>=2.19.1',
     'six>=1.11.0',
     'securesystemslib>=0.18.0'
   ],
-  tests_require = [
-    'mock; python_version < "3.3"'
-  ],
   packages = find_packages(exclude=['tests']),
   scripts = [
     'tuf/scripts/repo.py',

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = lint,py{27,36,37,38,39}
+envlist = lint,py{36,37,38,39}
 skipsdist = true
 
 [testenv]