Use static return type in OTPHP\OTP (#265)

The new immutable interface was leading to type confusion, by having
the common methods in OTP return static instead of self, PHP and
static analyzers like PHPStan will understand that they are still
working with a TOTP or HOTP class, rather than get confused and
think we're operating on the underlying OTP class.

Closes: #263

Author: anthonyryan1

src/OTP.php

@@ -102,7 +102,7 @@ public function setLabel(string $label): void
         $this->setParameter('label', $label);
     }
 
-    public function withLabel(string $label): self
+    public function withLabel(string $label): static
     {
         $otp = clone $this;
         $otp->setParameter('label', $label);
@@ -120,7 +120,7 @@ public function setIssuer(string $issuer): void
         $this->setParameter('issuer', $issuer);
     }
 
-    public function withIssuer(string $issuer): self
+    public function withIssuer(string $issuer): static
     {
         $otp = clone $this;
         $otp->setParameter('issuer', $issuer);
@@ -138,7 +138,7 @@ public function setIssuerIncludedAsParameter(bool $issuer_included_as_parameter)
         $this->issuer_included_as_parameter = $issuer_included_as_parameter;
     }
 
-    public function withIssuerIncludedAsParameter(bool $issuer_included_as_parameter): self
+    public function withIssuerIncludedAsParameter(bool $issuer_included_as_parameter): static
     {
         $otp = clone $this;
         $otp->issuer_included_as_parameter = $issuer_included_as_parameter;
@@ -200,7 +200,7 @@ public function setParameter(string $parameter, mixed $value): void
         }
     }
 
-    public function withParameter(string $parameter, mixed $value): self
+    public function withParameter(string $parameter, mixed $value): static
     {
         $otp = clone $this;
         $otp->setParameter($parameter, $value);
@@ -213,7 +213,7 @@ public function setSecret(string $secret): void
         $this->setParameter('secret', $secret);
     }
 
-    public function withSecret(string $secret): self
+    public function withSecret(string $secret): static
     {
         $otp = clone $this;
         $otp->setParameter('secret', $secret);
@@ -226,7 +226,7 @@ public function setDigits(int $digits): void
         $this->setParameter('digits', $digits);
     }
 
-    public function withDigits(int $digits): self
+    public function withDigits(int $digits): static
     {
         $otp = clone $this;
         $otp->setParameter('digits', $digits);
@@ -239,7 +239,7 @@ public function setDigest(string $digest): void
         $this->setParameter('algorithm', $digest);
     }
 
-    public function withDigest(string $digest): self
+    public function withDigest(string $digest): static
     {
         $otp = clone $this;
         $otp->setParameter('algorithm', $digest);