style: clang-tidy: default checks and fix bug in iostream deconstruction

```
/pybind11/include/pybind11/iostream.h:71:9: warning: Call to virtual method 'pythonbuf::sync' during destruction bypasses virtual dispatch [clang-analyzer-optin.cplusplus.VirtualCall]
        sync();
        ^
/pybind11/tests/test_iostream.cpp:72:5: note: Calling '~scoped_ostream_redirect'
    });
```

Author: henryiii

.clang-tidy

@@ -1,7 +1,6 @@
 FormatStyle: file
 
 Checks: '
--*,
 llvm-namespace-comment,
 modernize-use-override,
 readability-container-size-empty,

include/pybind11/iostream.h

@@ -38,7 +38,8 @@ class pythonbuf : public std::streambuf {
         return sync() == 0 ? traits_type::not_eof(c) : traits_type::eof();
     }
 
-    int sync() override {
+    // This function must be non-virtual to be called in a destructor
+    int _sync() {
         if (pbase() != pptr()) {
             // This subtraction cannot be negative, so dropping the sign
             str line(pbase(), static_cast<size_t>(pptr() - pbase()));
@@ -54,6 +55,10 @@ class pythonbuf : public std::streambuf {
         return 0;
     }
 
+    int sync() override {
+        return _sync();
+    }
+
 public:
 
     pythonbuf(object pyostream, size_t buffer_size = 1024)
@@ -68,7 +73,7 @@ class pythonbuf : public std::streambuf {
 
     /// Sync before destroy
     ~pythonbuf() override {
-        sync();
+        _sync();
     }
 };