Use Python 3.8 for pipenv

Python 3.8.0 entered Arch Linux core repositories now and all tests
pass with Python 3.8.0 so it's time...

Pipfile

@@ -23,7 +23,7 @@ csv-metadata-quality = {editable = true,path = "."}
 langid = "*"
 
 [requires]
-python_version = "3.7"
+python_version = "3.8"
 
 [pipenv]
 allow_prereleases = true

Pipfile.lock

@@ -1,11 +1,11 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "59562d8c59eb09e23b49475d6901687edbf605f5b84e283e90cc8e2de518641f"
+            "sha256": "00609ca58a66c6da9f80bd36e37622ea434ac80f75db749e04cf5a55a06b5dde"
         },
         "pipfile-spec": 6,
         "requires": {
-            "python_version": "3.7"
+            "python_version": "3.8"
         },
         "sources": [
             {
@@ -291,14 +291,6 @@
             ],
             "version": "==0.18.2"
         },
-        "importlib-metadata": {
-            "hashes": [
-                "sha256:aa18d7378b00b40847790e7c27e11673d7fed219354109d0e7b9e5b25dc3ad26",
-                "sha256:d5f18a79777f3aa179c145737780282e27b508fc8fd688cb17c7a813e8bd39af"
-            ],
-            "markers": "python_version < '3.8'",
-            "version": "==0.23"
-        },
         "ipython": {
             "hashes": [
                 "sha256:dfd303b270b7b5232b3d08bd30ec6fd685d8a58cabd54055e3d69d8f029f7280",
@@ -366,9 +358,9 @@
         },
         "openpyxl": {
             "hashes": [
-                "sha256:340a1ab2069764559b9d58027a43a24db18db0e25deb80f81ecb8ca7ee5253db"
+                "sha256:a3ee361d3ff04af6048d594775b3a54ffdf215d40fa5c6c78b2a41c0d0b020d3"
             ],
-            "version": "==3.0.0"
+            "version": "==3.0.1"
         },
         "packaging": {
             "hashes": [
@@ -601,13 +593,6 @@
             ],
             "index": "pypi",
             "version": "==1.2.0"
-        },
-        "zipp": {
-            "hashes": [
-                "sha256:3718b1cbcd963c7d4c5511a8240812904164b7f381b647143a89d3b98f9bcd8e",
-                "sha256:f06903e9f1f43b12d371004b4ac7b06ab39a44adc747266928ae6debfa7b3335"
-            ],
-            "version": "==0.6.0"
         }
     }
 }