chore(deps): bump skyfield from 1.20 to 1.21

Bumps [skyfield](https://github.com/brandon-rhodes/python-skyfield) from 1.20 to 1.21.
- [Release notes](https://github.com/brandon-rhodes/python-skyfield/releases)
- [Changelog](https://github.com/skyfielders/python-skyfield/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/brandon-rhodes/python-skyfield/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Pipfile.lock

@@ -99,38 +99,38 @@
         },
         "sgp4": {
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+            ],
+            "version": "==2.12"
         },
         "shutilwhich": {
             "hashes": [
@@ -140,17 +140,17 @@
         },
         "six": {
             "hashes": [
-                "sha256:236bdbdce46e6e6a3d61a337c0f8b763ca1e8717c03b369e87a7ec7ce1319c0a",
-                "sha256:8f3cd2e254d8f793e7f3d6d9df77b92252b52637291d0f0da013c76ea2724b6c"
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             ],
-            "version": "==1.14.0"
+            "version": "==1.15.0"
         },
         "skyfield": {
             "hashes": [
-                "sha256:5dbc4626714c32471524e49b0496806dd87366cbe295ed1e077c6fd5be91a452"
+                "sha256:14991dde6464dfa1c34479f6b274c07f2e84323b3a751faed62dd676b7bf69ce"
             ],
             "index": "pypi",
-            "version": "==1.20"
+            "version": "==1.21"
         },
         "tabulate": {
             "hashes": [
@@ -326,10 +326,10 @@
         },
         "six": {
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             ],
-            "version": "==1.14.0"
+            "version": "==1.15.0"
         },
         "toml": {
             "hashes": [