Add flake8 to pipenv dev environment

To help check PEP8 formatting/style compliance.

.flake8

@@ -0,0 +1,2 @@
+[flake8]
+ignore = E501

Pipfile

@@ -6,6 +6,7 @@ verify_ssl = true
 [dev-packages]
 pytest = "*"
 ipython = "*"
+flake8 = "*"
 
 [packages]
 pandas = "*"

Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "c98ad40f5fc15aaa49f63af944091f7a11cf3d0ee1184f4e9dd927913155c011"
+            "sha256": "5fa81e553c4f7f25d8e05b1ffec5d65f6661c2351274bca97fb50041bd84f9c5"
         },
         "pipfile-spec": 6,
         "requires": {
@@ -124,6 +124,21 @@
             ],
             "version": "==4.4.0"
         },
+        "entrypoints": {
+            "hashes": [
+                "sha256:589f874b313739ad35be6e0cd7efde2a4e9b6fea91edcc34e58ecbb8dbe56d19",
+                "sha256:c70dd71abe5a8c85e55e12c19bd91ccfeec11a6e99044204511f9ed547d48451"
+            ],
+            "version": "==0.3"
+        },
+        "flake8": {
+            "hashes": [
+                "sha256:19241c1cbc971b9962473e4438a2ca19749a7dd002dd1a946eaba171b4114548",
+                "sha256:8e9dfa3cecb2400b3738a42c54c3043e821682b9c840b0448c0503f781130696"
+            ],
+            "index": "pypi",
+            "version": "==3.7.8"
+        },
         "importlib-metadata": {
             "hashes": [
                 "sha256:6dfd58dfe281e8d240937776065dd3624ad5469c835248219bd16cf2e12dbeb7",
@@ -153,6 +168,13 @@
             ],
             "version": "==0.14.1"
         },
+        "mccabe": {
+            "hashes": [
+                "sha256:ab8a6258860da4b6677da4bd2fe5dc2c659cff31b3ee4f7f5d64e79735b80d42",
+                "sha256:dd8d182285a0fe56bace7f45b5e7d1a6ebcbf524e8f3bd87eb0f125271b8831f"
+            ],
+            "version": "==0.6.1"
+        },
         "more-itertools": {
             "hashes": [
                 "sha256:409cd48d4db7052af495b09dec721011634af3753ae1ef92d2b32f73a745f832",
@@ -218,6 +240,20 @@
             ],
             "version": "==1.8.0"
         },
+        "pycodestyle": {
+            "hashes": [
+                "sha256:95a2219d12372f05704562a14ec30bc76b05a5b297b21a5dfe3f6fac3491ae56",
+                "sha256:e40a936c9a450ad81df37f549d676d127b1b66000a6c500caa2b085bc0ca976c"
+            ],
+            "version": "==2.5.0"
+        },
+        "pyflakes": {
+            "hashes": [
+                "sha256:17dbeb2e3f4d772725c777fabc446d5634d1038f234e77343108ce445ea69ce0",
+                "sha256:d976835886f8c5b31d47970ed689944a0262b5f3afa00a5a7b4dc81e5449f8a2"
+            ],
+            "version": "==2.1.1"
+        },
         "pygments": {
             "hashes": [
                 "sha256:71e430bc85c88a430f000ac1d9b331d2407f681d6f6aec95e8bcfbc3df5b0127",