Master subnet validations (patched)

[shashwat1002]

Patches the work done in pull request #54 , so that it works for python 3.5 and also sorts out certain flake8 issues.

[coveralls]

Pull Request Test Coverage Report for Build 193

• 17 of 20 (85.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.6%) to 99.355%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
django_ipam/apps.py	14	17	82.35%
<!--	Total:	17	20	85.0%	-->

Totals
Change from base Build 192:	-0.6%
Covered Lines:	462
Relevant Lines:	465

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 194

• 20 of 23 (86.96%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.6%) to 99.359%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
django_ipam/compat.py	14	17	82.35%
<!--	Total:	20	23	86.96%	-->

Totals
Change from base Build 192:	-0.6%
Covered Lines:	465
Relevant Lines:	468

---

💛 - Coveralls

[nemesifier]

@shashwat1002 great work, thanks! :+1:

See my improvements for future reference: https://github.com/openwisp/django-ipam/commit/a74ab5d115c2421007e3491a50394f6dac93af06