Rename master branch to main

There fortunately seem to be very few mentions of the default branch in
the source code.

INSTALL

@@ -28,7 +28,7 @@ installed using the pip installer::
 
     pip install cartopy
 
-To instead install the most recent version found on the GitHub master branch,
+To instead install the most recent version found on the GitHub main branch,
 use::
 
     pip install git+https://github.com/SciTools/cartopy.git

README.md

@@ -17,14 +17,14 @@
 <a href="https://github.com/SciTools/cartopy/releases">
 <img src="https://img.shields.io/github/tag/SciTools/cartopy.svg"
  alt="Latest version" /></a>
-<a href="https://github.com/SciTools/cartopy/commits/master">
+<a href="https://github.com/SciTools/cartopy/commits/main">
 <img src="https://img.shields.io/github/commits-since/SciTools/cartopy/latest.svg"
  alt="Commits since last release" /></a>
 <a href="https://github.com/SciTools/cartopy/graphs/contributors">
 <img src="https://img.shields.io/github/contributors/SciTools/cartopy.svg"
  alt="# contributors" /></a>
 <a href="https://travis-ci.org/SciTools/cartopy/branches">
-<img src="https://api.travis-ci.org/repositories/SciTools/cartopy.svg?branch=master"
+<img src="https://api.travis-ci.org/repositories/SciTools/cartopy.svg?branch=main"
  alt="Travis-CI" /></a>
 <a href="https://zenodo.org/badge/latestdoi/5282596">
 <img src="https://zenodo.org/badge/5282596.svg"

benchmarks/asv.conf.json

@@ -15,7 +15,7 @@
 
     // List of branches to benchmark. If not provided, defaults to "master"
     // (for git) or "default" (for mercurial).
-    // "branches": ["master"], // for git
+    "branches": ["main"],
 
     // The tool to use to create environments.  May be "conda",
     // "virtualenv" or other value depending on the plugins in use.

docs/source/conf.py

@@ -65,8 +65,8 @@
 # The encoding of source files.
 # source_encoding = 'utf-8-sig'
 
-# The master toctree document.
-master_doc = 'index'
+# The root toctree document (variable renamed in Sphinx 4.)
+root_doc = master_doc = 'index'
 
 # General information about the project.
 project = 'cartopy'