ci: add missing dep

Pipfile

@@ -15,6 +15,7 @@ coverage = "==6.0.1"
 codecov = "==2.1.12"
 pytest = "==6.2.5"
 black = "===21.9b0"
+flake8 = "===3.7.9"
 
 [requires]
 python_version = "3.9"

Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "85609d2dddc8429567481e65ffe1f49f5df31002d2d8f60bd29c6cd499d4cad4"
+            "sha256": "75fa47a603ce4c5f14f1a98b7654b147f1e25197409d4b0e5c0ded351aae84c6"
         },
         "pipfile-spec": 6,
         "requires": {
@@ -218,6 +218,22 @@
             "index": "pypi",
             "version": "==6.0.1"
         },
+        "entrypoints": {
+            "hashes": [
+                "sha256:589f874b313739ad35be6e0cd7efde2a4e9b6fea91edcc34e58ecbb8dbe56d19",
+                "sha256:c70dd71abe5a8c85e55e12c19bd91ccfeec11a6e99044204511f9ed547d48451"
+            ],
+            "markers": "python_version >= '2.7'",
+            "version": "==0.3"
+        },
+        "flake8": {
+            "hashes": [
+                "sha256:45681a117ecc81e870cbf1262835ae4af5e7a8b08e40b944a8a6e6b895914cfb",
+                "sha256:49356e766643ad15072a789a20915d3c91dc89fd313ccd71802303fd67e4deca"
+            ],
+            "index": "pypi",
+            "version": "===3.7.9"
+        },
         "idna": {
             "hashes": [
                 "sha256:14475042e284991034cb48e06f6851428fb14c4dc953acd9be9a5e95c7b6dd7a",
@@ -233,6 +249,13 @@
             ],
             "version": "==1.1.1"
         },
+        "mccabe": {
+            "hashes": [
+                "sha256:ab8a6258860da4b6677da4bd2fe5dc2c659cff31b3ee4f7f5d64e79735b80d42",
+                "sha256:dd8d182285a0fe56bace7f45b5e7d1a6ebcbf524e8f3bd87eb0f125271b8831f"
+            ],
+            "version": "==0.6.1"
+        },
         "mypy-extensions": {
             "hashes": [
                 "sha256:090fedd75945a69ae91ce1303b5824f428daf5a028d2f6ab8a299250a846f15d",
@@ -279,6 +302,22 @@
             "markers": "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'",
             "version": "==1.10.0"
         },
+        "pycodestyle": {
+            "hashes": [
+                "sha256:95a2219d12372f05704562a14ec30bc76b05a5b297b21a5dfe3f6fac3491ae56",
+                "sha256:e40a936c9a450ad81df37f549d676d127b1b66000a6c500caa2b085bc0ca976c"
+            ],
+            "markers": "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'",
+            "version": "==2.5.0"
+        },
+        "pyflakes": {
+            "hashes": [
+                "sha256:17dbeb2e3f4d772725c777fabc446d5634d1038f234e77343108ce445ea69ce0",
+                "sha256:d976835886f8c5b31d47970ed689944a0262b5f3afa00a5a7b4dc81e5449f8a2"
+            ],
+            "markers": "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3'",
+            "version": "==2.1.1"
+        },
         "pyparsing": {
             "hashes": [
                 "sha256:c203ec8783bf771a155b207279b9bccb8dea02d8f0c9e5f8ead507bc3246ecc1",